coccinelle: alloc_cast: drop removed allocatorsgrafted

- pci_alloc_consistent() and pci_zalloc_consistent() were removed by
  commit 7968778914e5
  ("PCI: Remove the deprecated "pci-dma-compat.h" API")

- kmem_alloc() was removed by commit f078d4ea8276
  ("xfs: convert kmem_alloc() to kmalloc()")

- kmem_zalloc() was removed by commit 10634530f7ba
  ("xfs: convert kmem_zalloc() to kzalloc()")

- kmem_zone_alloc() and kmem_zone_zalloc() were removed by commit
  bae633a4a283 ("xfs: remove xfs_zone_{alloc,zalloc} helpers")

So drop them from the rules.

No functional change.

Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>

1 files changed, 4886 insertions, 0 deletions

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
new file mode 100644
index 000000000..de4cc2483
--- /dev/null
+++ b/mm/userfaultfd.c
@@ -0,0 +1,4886 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ *  mm/userfaultfd.c
+ *
+ *  Copyright (C) 2007  Davide Libenzi <davidel@xmailserver.org>
+ *  Copyright (C) 2008-2009 Red Hat, Inc.
+ *  Copyright (C) 2015  Red Hat, Inc.
+ *
+ *  Some part derived from fs/eventfd.c (anon inode setup) and
+ *  mm/ksm.c (mm hashing).
+ */
+
+#include <linux/mm.h>
+#include <linux/sched/signal.h>
+#include <linux/pagemap.h>
+#include <linux/rmap.h>
+#include <linux/swap.h>
+#include <linux/leafops.h>
+#include <linux/userfaultfd_k.h>
+#include <linux/mmu_notifier.h>
+#include <linux/hugetlb.h>
+#include <linux/list.h>
+#include <linux/sched/mm.h>
+#include <linux/mm_inline.h>
+#include <linux/poll.h>
+#include <linux/slab.h>
+#include <linux/seq_file.h>
+#include <linux/bug.h>
+#include <linux/anon_inodes.h>
+#include <linux/syscalls.h>
+#include <linux/miscdevice.h>
+#include <linux/uio.h>
+#include <linux/file.h>
+#include <linux/cleanup.h>
+#include <asm/tlbflush.h>
+#include <asm/tlb.h>
+#include "internal.h"
+#include "swap.h"
+
+struct mfill_state {
+	struct userfaultfd_ctx *ctx;
+	unsigned long src_start;
+	unsigned long dst_start;
+	unsigned long len;
+	uffd_flags_t flags;
+
+	struct vm_area_struct *vma;
+	unsigned long src_addr;
+	unsigned long dst_addr;
+	pmd_t *pmd;
+};
+
+static bool anon_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags)
+{
+	/* anonymous memory does not support MINOR mode */
+	if (vm_flags & VM_UFFD_MINOR)
+		return false;
+	return true;
+}
+
+static struct folio *anon_alloc_folio(struct vm_area_struct *vma,
+				      unsigned long addr)
+{
+	struct folio *folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, vma,
+					      addr);
+
+	if (!folio)
+		return NULL;
+
+	if (mem_cgroup_charge(folio, vma->vm_mm, GFP_KERNEL)) {
+		folio_put(folio);
+		return NULL;
+	}
+
+	return folio;
+}
+
+static const struct vm_uffd_ops anon_uffd_ops = {
+	.can_userfault	= anon_can_userfault,
+	.alloc_folio	= anon_alloc_folio,
+};
+
+static const struct vm_uffd_ops *vma_uffd_ops(struct vm_area_struct *vma)
+{
+	if (vma_is_anonymous(vma))
+		return &anon_uffd_ops;
+	return vma->vm_ops->uffd_ops;
+}
+
+static __always_inline
+bool validate_dst_vma(struct vm_area_struct *dst_vma, unsigned long dst_end)
+{
+	/* Make sure that the dst range is fully within dst_vma. */
+	if (dst_end > dst_vma->vm_end)
+		return false;
+
+	/*
+	 * Check the vma is registered in uffd, this is required to
+	 * enforce the VM_MAYWRITE check done at uffd registration
+	 * time.
+	 */
+	if (!dst_vma->vm_userfaultfd_ctx.ctx)
+		return false;
+
+	return true;
+}
+
+static __always_inline
+struct vm_area_struct *find_vma_and_prepare_anon(struct mm_struct *mm,
+						 unsigned long addr)
+{
+	struct vm_area_struct *vma;
+
+	mmap_assert_locked(mm);
+	vma = vma_lookup(mm, addr);
+	if (!vma)
+		vma = ERR_PTR(-ENOENT);
+	else if (!(vma->vm_flags & VM_SHARED) &&
+		 unlikely(anon_vma_prepare(vma)))
+		vma = ERR_PTR(-ENOMEM);
+
+	return vma;
+}
+
+#ifdef CONFIG_PER_VMA_LOCK
+/*
+ * uffd_lock_vma() - Lookup and lock vma corresponding to @address.
+ * @mm: mm to search vma in.
+ * @address: address that the vma should contain.
+ *
+ * Should be called without holding mmap_lock.
+ *
+ * Return: A locked vma containing @address, -ENOENT if no vma is found, or
+ * -ENOMEM if anon_vma couldn't be allocated.
+ */
+static struct vm_area_struct *uffd_lock_vma(struct mm_struct *mm,
+				       unsigned long address)
+{
+	struct vm_area_struct *vma;
+
+	vma = lock_vma_under_rcu(mm, address);
+	if (vma) {
+		/*
+		 * We know we're going to need to use anon_vma, so check
+		 * that early.
+		 */
+		if (!(vma->vm_flags & VM_SHARED) && unlikely(!vma->anon_vma))
+			vma_end_read(vma);
+		else
+			return vma;
+	}
+
+	mmap_read_lock(mm);
+	vma = find_vma_and_prepare_anon(mm, address);
+	if (!IS_ERR(vma)) {
+		bool locked = vma_start_read_locked(vma);
+
+		if (!locked)
+			vma = ERR_PTR(-EAGAIN);
+	}
+
+	mmap_read_unlock(mm);
+	return vma;
+}
+
+static struct vm_area_struct *uffd_mfill_lock(struct mm_struct *dst_mm,
+					      unsigned long dst_start,
+					      unsigned long len)
+{
+	struct vm_area_struct *dst_vma;
+
+	dst_vma = uffd_lock_vma(dst_mm, dst_start);
+	if (IS_ERR(dst_vma) || validate_dst_vma(dst_vma, dst_start + len))
+		return dst_vma;
+
+	vma_end_read(dst_vma);
+	return ERR_PTR(-ENOENT);
+}
+
+static void uffd_mfill_unlock(struct vm_area_struct *vma)
+{
+	vma_end_read(vma);
+}
+
+#else
+
+static struct vm_area_struct *uffd_mfill_lock(struct mm_struct *dst_mm,
+					      unsigned long dst_start,
+					      unsigned long len)
+{
+	struct vm_area_struct *dst_vma;
+
+	mmap_read_lock(dst_mm);
+	dst_vma = find_vma_and_prepare_anon(dst_mm, dst_start);
+	if (IS_ERR(dst_vma))
+		goto out_unlock;
+
+	if (validate_dst_vma(dst_vma, dst_start + len))
+		return dst_vma;
+
+	dst_vma = ERR_PTR(-ENOENT);
+out_unlock:
+	mmap_read_unlock(dst_mm);
+	return dst_vma;
+}
+
+static void uffd_mfill_unlock(struct vm_area_struct *vma)
+{
+	mmap_read_unlock(vma->vm_mm);
+}
+#endif
+
+static void mfill_put_vma(struct mfill_state *state)
+{
+	if (!state->vma)
+		return;
+
+	up_read(&state->ctx->map_changing_lock);
+	uffd_mfill_unlock(state->vma);
+	state->vma = NULL;
+}
+
+static int mfill_get_vma(struct mfill_state *state)
+{
+	struct userfaultfd_ctx *ctx = state->ctx;
+	uffd_flags_t flags = state->flags;
+	struct vm_area_struct *dst_vma;
+	const struct vm_uffd_ops *ops;
+	int err;
+
+	/*
+	 * Make sure the vma is not shared, that the dst range is
+	 * both valid and fully within a single existing vma.
+	 */
+	dst_vma = uffd_mfill_lock(ctx->mm, state->dst_start, state->len);
+	if (IS_ERR(dst_vma))
+		return PTR_ERR(dst_vma);
+
+	/*
+	 * If memory mappings are changing because of non-cooperative
+	 * operation (e.g. mremap) running in parallel, bail out and
+	 * request the user to retry later
+	 */
+	down_read(&ctx->map_changing_lock);
+	state->vma = dst_vma;
+	err = -EAGAIN;
+	if (atomic_read(&ctx->mmap_changing))
+		goto out_unlock;
+
+	err = -EINVAL;
+
+	/*
+	 * shmem_zero_setup is invoked in mmap for MAP_ANONYMOUS|MAP_SHARED but
+	 * it will overwrite vm_ops, so vma_is_anonymous must return false.
+	 */
+	if (WARN_ON_ONCE(vma_is_anonymous(dst_vma) &&
+	    dst_vma->vm_flags & VM_SHARED))
+		goto out_unlock;
+
+	/*
+	 * validate 'mode' now that we know the dst_vma: don't allow
+	 * a wrprotect copy if the userfaultfd didn't register as WP.
+	 */
+	if ((flags & MFILL_ATOMIC_WP) && !(dst_vma->vm_flags & VM_UFFD_WP))
+		goto out_unlock;
+
+	if (is_vm_hugetlb_page(dst_vma))
+		return 0;
+
+	ops = vma_uffd_ops(dst_vma);
+	if (!ops)
+		goto out_unlock;
+
+	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE) &&
+	    !ops->get_folio_noalloc)
+		goto out_unlock;
+
+	return 0;
+
+out_unlock:
+	mfill_put_vma(state);
+	return err;
+}
+
+static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
+{
+	pgd_t *pgd;
+	p4d_t *p4d;
+	pud_t *pud;
+
+	pgd = pgd_offset(mm, address);
+	p4d = p4d_alloc(mm, pgd, address);
+	if (!p4d)
+		return NULL;
+	pud = pud_alloc(mm, p4d, address);
+	if (!pud)
+		return NULL;
+	/*
+	 * Note that we didn't run this because the pmd was
+	 * missing, the *pmd may be already established and in
+	 * turn it may also be a trans_huge_pmd.
+	 */
+	return pmd_alloc(mm, pud, address);
+}
+
+static int mfill_establish_pmd(struct mfill_state *state)
+{
+	struct mm_struct *dst_mm = state->ctx->mm;
+	pmd_t *dst_pmd, dst_pmdval;
+
+	dst_pmd = mm_alloc_pmd(dst_mm, state->dst_addr);
+	if (unlikely(!dst_pmd))
+		return -ENOMEM;
+
+	dst_pmdval = pmdp_get_lockless(dst_pmd);
+	if (unlikely(pmd_none(dst_pmdval)) &&
+	    unlikely(__pte_alloc(dst_mm, dst_pmd)))
+		return -ENOMEM;
+
+	dst_pmdval = pmdp_get_lockless(dst_pmd);
+	/*
+	 * If the dst_pmd is THP don't override it and just be strict.
+	 * (This includes the case where the PMD used to be THP and
+	 * changed back to none after __pte_alloc().)
+	 */
+	if (unlikely(!pmd_present(dst_pmdval) || pmd_leaf(dst_pmdval)))
+		return -EEXIST;
+	if (unlikely(pmd_bad(dst_pmdval)))
+		return -EFAULT;
+
+	state->pmd = dst_pmd;
+	return 0;
+}
+
+/* Check if dst_addr is outside of file's size. Must be called with ptl held. */
+static bool mfill_file_over_size(struct vm_area_struct *dst_vma,
+				 unsigned long dst_addr)
+{
+	struct inode *inode;
+	pgoff_t offset, max_off;
+
+	if (!dst_vma->vm_file)
+		return false;
+
+	inode = dst_vma->vm_file->f_inode;
+	offset = linear_page_index(dst_vma, dst_addr);
+	max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
+	return offset >= max_off;
+}
+
+/*
+ * Install PTEs, to map dst_addr (within dst_vma) to page.
+ *
+ * This function handles both MCOPY_ATOMIC_NORMAL and _CONTINUE for both shmem
+ * and anon, and for both shared and private VMAs.
+ */
+static int mfill_atomic_install_pte(pmd_t *dst_pmd,
+				    struct vm_area_struct *dst_vma,
+				    unsigned long dst_addr, struct page *page,
+				    uffd_flags_t flags)
+{
+	int ret;
+	struct mm_struct *dst_mm = dst_vma->vm_mm;
+	pte_t _dst_pte, *dst_pte;
+	bool writable = dst_vma->vm_flags & VM_WRITE;
+	bool vm_shared = dst_vma->vm_flags & VM_SHARED;
+	spinlock_t *ptl;
+	struct folio *folio = page_folio(page);
+	bool page_in_cache = folio_mapping(folio);
+	pte_t dst_ptep;
+
+	_dst_pte = mk_pte(page, dst_vma->vm_page_prot);
+	_dst_pte = pte_mkdirty(_dst_pte);
+	if (page_in_cache && !vm_shared)
+		writable = false;
+	if (writable)
+		_dst_pte = pte_mkwrite(_dst_pte, dst_vma);
+	if (flags & MFILL_ATOMIC_WP)
+		_dst_pte = pte_mkuffd(_dst_pte);
+
+	ret = -EAGAIN;
+	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
+	if (!dst_pte)
+		goto out;
+
+	if (mfill_file_over_size(dst_vma, dst_addr)) {
+		ret = -EFAULT;
+		goto out_unlock;
+	}
+
+	ret = -EEXIST;
+
+	dst_ptep = ptep_get(dst_pte);
+
+	/*
+	 * We are allowed to overwrite a UFFD pte marker: consider when both
+	 * MISSING|WP registered, we firstly wr-protect a none pte which has no
+	 * page cache page backing it, then access the page.
+	 */
+	if (!pte_none(dst_ptep) && !pte_is_uffd_marker(dst_ptep))
+		goto out_unlock;
+
+	if (page_in_cache) {
+		folio_add_file_rmap_pte(folio, page, dst_vma);
+	} else {
+		folio_add_new_anon_rmap(folio, dst_vma, dst_addr, RMAP_EXCLUSIVE);
+		folio_add_lru_vma(folio, dst_vma);
+	}
+
+	/*
+	 * Must happen after rmap, as mm_counter() checks mapping (via
+	 * PageAnon()), which is set by __page_set_anon_rmap().
+	 */
+	inc_mm_counter(dst_mm, mm_counter(folio));
+
+	set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
+
+	if (page_in_cache)
+		folio_unlock(folio);
+
+	/* No need to invalidate - it was non-present before */
+	update_mmu_cache(dst_vma, dst_addr, dst_pte);
+	ret = 0;
+out_unlock:
+	pte_unmap_unlock(dst_pte, ptl);
+out:
+	return ret;
+}
+
+static int mfill_copy_folio_locked(struct folio *folio, unsigned long src_addr)
+{
+	void *kaddr;
+	int ret;
+
+	kaddr = kmap_local_folio(folio, 0);
+	/*
+	 * The read mmap_lock is held here.  Despite the
+	 * mmap_lock being read recursive a deadlock is still
+	 * possible if a writer has taken a lock.  For example:
+	 *
+	 * process A thread 1 takes read lock on own mmap_lock
+	 * process A thread 2 calls mmap, blocks taking write lock
+	 * process B thread 1 takes page fault, read lock on own mmap lock
+	 * process B thread 2 calls mmap, blocks taking write lock
+	 * process A thread 1 blocks taking read lock on process B
+	 * process B thread 1 blocks taking read lock on process A
+	 *
+	 * Disable page faults to prevent potential deadlock
+	 * and retry the copy outside the mmap_lock.
+	 */
+	pagefault_disable();
+	ret = copy_from_user(kaddr, (const void __user *) src_addr,
+			     PAGE_SIZE);
+	pagefault_enable();
+	kunmap_local(kaddr);
+
+	if (ret)
+		return -EFAULT;
+
+	flush_dcache_folio(folio);
+	return ret;
+}
+
+#define MFILL_RETRY_STATE_VMA_FLAGS \
+	append_vma_flags(__VMA_UFFD_FLAGS, VMA_SHARED_BIT)
+
+/*
+ * VMA state saved before dropping the locks in mfill_copy_folio_retry().
+ * Used to detect VMA replacement or incompatible changes after reacquiring the
+ * locks.
+ */
+struct mfill_retry_state {
+	const struct vm_uffd_ops *ops;
+	struct file *file;
+	vma_flags_t flags;
+	pgoff_t pgoff;
+};
+
+static void mfill_retry_state_save(struct mfill_retry_state *s,
+				   struct vm_area_struct *vma)
+{
+	s->flags = vma_flags_and_mask(&vma->flags, MFILL_RETRY_STATE_VMA_FLAGS);
+	s->ops = vma_uffd_ops(vma);
+	s->pgoff = vma_start_pgoff(vma);
+
+	if (vma->vm_file)
+		s->file = get_file(vma->vm_file);
+}
+
+static bool mfill_retry_state_changed(struct mfill_retry_state *state,
+				      struct vm_area_struct *vma)
+{
+	vma_flags_t flags = vma_flags_and_mask(&vma->flags,
+					       MFILL_RETRY_STATE_VMA_FLAGS);
+
+	/* Have any UFFD flags (missing, WP, minor) changed? */
+	if (!vma_flags_same_pair(&state->flags, &flags))
+		return true;
+
+	/* VMA type or effective uffd_ops changed while the lock was dropped */
+	if (state->ops != vma_uffd_ops(vma))
+		return true;
+
+	/* VMA was anonymous before; changed only if it no longer is */
+	if (!state->file)
+		return !vma_is_anonymous(vma);
+
+	/* VMA was file backed, but file, inode or offset has changed */
+	if (!vma->vm_file || vma->vm_file->f_inode != state->file->f_inode ||
+	    state->file != vma->vm_file || vma_start_pgoff(vma) != state->pgoff)
+		return true;
+
+	return false;
+}
+
+static void mfill_retry_state_put(struct mfill_retry_state *s)
+{
+	if (s->file)
+		fput(s->file);
+}
+
+DEFINE_FREE(retry_put, struct mfill_retry_state *,
+	    if (_T) mfill_retry_state_put(_T));
+
+static int mfill_copy_folio_retry(struct mfill_state *mfill_state,
+				  struct folio *folio)
+{
+	struct mfill_retry_state retry_state = { 0 };
+	struct mfill_retry_state *for_free __free(retry_put) = &retry_state;
+	unsigned long src_addr = mfill_state->src_addr;
+	void *kaddr;
+	int err;
+
+	mfill_retry_state_save(&retry_state, mfill_state->vma);
+
+	/* retry copying with mm_lock dropped */
+	mfill_put_vma(mfill_state);
+
+	kaddr = kmap_local_folio(folio, 0);
+	err = copy_from_user(kaddr, (const void __user *) src_addr, PAGE_SIZE);
+	kunmap_local(kaddr);
+	if (unlikely(err))
+		return -EFAULT;
+
+	flush_dcache_folio(folio);
+
+	/* reget VMA and PMD, they could change underneath us */
+	err = mfill_get_vma(mfill_state);
+	if (err)
+		return err;
+
+	if (mfill_retry_state_changed(&retry_state, mfill_state->vma))
+		return -EAGAIN;
+
+	err = mfill_establish_pmd(mfill_state);
+	if (err)
+		return err;
+
+	return 0;
+}
+
+static int __mfill_atomic_pte(struct mfill_state *state,
+			      const struct vm_uffd_ops *ops)
+{
+	unsigned long dst_addr = state->dst_addr;
+	unsigned long src_addr = state->src_addr;
+	uffd_flags_t flags = state->flags;
+	struct folio *folio;
+	int ret;
+
+	if (!ops) {
+		VM_WARN_ONCE(1, "UFFDIO_COPY for unsupported VMA");
+		return -EOPNOTSUPP;
+	}
+
+	folio = ops->alloc_folio(state->vma, state->dst_addr);
+	if (!folio)
+		return -ENOMEM;
+
+	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_COPY)) {
+		ret = mfill_copy_folio_locked(folio, src_addr);
+		/*
+		 * Fallback to copy_from_user outside mmap_lock.
+		 * If retry is successful, mfill_copy_folio_locked() returns
+		 * with locks retaken by mfill_get_vma().
+		 * If there was an error, we must mfill_put_vma() anyway and it
+		 * will take care of unlocking if needed.
+		 */
+		if (unlikely(ret)) {
+			ret = mfill_copy_folio_retry(state, folio);
+			if (ret)
+				goto err_folio_put;
+		}
+	} else if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE)) {
+		clear_user_highpage(&folio->page, state->dst_addr);
+	} else {
+		VM_WARN_ONCE(1, "Unknown UFFDIO operation, flags: %x", flags);
+	}
+
+	/*
+	 * The memory barrier inside __folio_mark_uptodate makes sure that
+	 * preceding stores to the page contents become visible before
+	 * the set_pte_at() write.
+	 */
+	__folio_mark_uptodate(folio);
+
+	if (ops->filemap_add) {
+		ret = ops->filemap_add(folio, state->vma, state->dst_addr);
+		if (ret)
+			goto err_folio_put;
+	}
+
+	ret = mfill_atomic_install_pte(state->pmd, state->vma, dst_addr,
+				       &folio->page, flags);
+	if (ret)
+		goto err_filemap_remove;
+
+	return 0;
+
+err_filemap_remove:
+	if (ops->filemap_remove)
+		ops->filemap_remove(folio, state->vma);
+err_folio_put:
+	folio_put(folio);
+	return ret;
+}
+
+static int mfill_atomic_pte_copy(struct mfill_state *state)
+{
+	const struct vm_uffd_ops *ops = vma_uffd_ops(state->vma);
+
+	/*
+	 * The normal page fault path for a MAP_PRIVATE mapping in a
+	 * file-backed VMA will invoke the fault, fill the hole in the file and
+	 * COW it right away. The result generates plain anonymous memory.
+	 * So when we are asked to fill a hole in a MAP_PRIVATE mapping, we'll
+	 * generate anonymous memory directly without actually filling the
+	 * hole. For the MAP_PRIVATE case the robustness check only happens in
+	 * the pagetable (to verify it's still none) and not in the page cache.
+	 */
+	if (!(state->vma->vm_flags & VM_SHARED))
+		ops = &anon_uffd_ops;
+
+	return __mfill_atomic_pte(state, ops);
+}
+
+static int mfill_atomic_pte_zeroed_folio(struct mfill_state *state)
+{
+	const struct vm_uffd_ops *ops = vma_uffd_ops(state->vma);
+
+	return __mfill_atomic_pte(state, ops);
+}
+
+static int mfill_atomic_pte_zeropage(struct mfill_state *state)
+{
+	struct vm_area_struct *dst_vma = state->vma;
+	unsigned long dst_addr = state->dst_addr;
+	pmd_t *dst_pmd = state->pmd;
+	pte_t _dst_pte, *dst_pte;
+	spinlock_t *ptl;
+	int ret;
+
+	if (mm_forbids_zeropage(dst_vma->vm_mm) ||
+	    (dst_vma->vm_flags & VM_SHARED))
+		return mfill_atomic_pte_zeroed_folio(state);
+
+	_dst_pte = pte_mkspecial(pfn_pte(zero_pfn(dst_addr),
+					 dst_vma->vm_page_prot));
+	ret = -EAGAIN;
+	dst_pte = pte_offset_map_lock(dst_vma->vm_mm, dst_pmd, dst_addr, &ptl);
+	if (!dst_pte)
+		goto out;
+	if (mfill_file_over_size(dst_vma, dst_addr)) {
+		ret = -EFAULT;
+		goto out_unlock;
+	}
+	ret = -EEXIST;
+	if (!pte_none(ptep_get(dst_pte)))
+		goto out_unlock;
+	set_pte_at(dst_vma->vm_mm, dst_addr, dst_pte, _dst_pte);
+	/* No need to invalidate - it was non-present before */
+	update_mmu_cache(dst_vma, dst_addr, dst_pte);
+	ret = 0;
+out_unlock:
+	pte_unmap_unlock(dst_pte, ptl);
+out:
+	return ret;
+}
+
+/* Handles UFFDIO_CONTINUE for all shmem VMAs (shared or private). */
+static int mfill_atomic_pte_continue(struct mfill_state *state)
+{
+	struct vm_area_struct *dst_vma = state->vma;
+	const struct vm_uffd_ops *ops = vma_uffd_ops(dst_vma);
+	unsigned long dst_addr = state->dst_addr;
+	pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);
+	struct inode *inode = file_inode(dst_vma->vm_file);
+	uffd_flags_t flags = state->flags;
+	pmd_t *dst_pmd = state->pmd;
+	struct folio *folio;
+	struct page *page;
+	int ret;
+
+	if (!ops) {
+		VM_WARN_ONCE(1, "UFFDIO_CONTINUE for unsupported VMA");
+		return -EOPNOTSUPP;
+	}
+
+	folio = ops->get_folio_noalloc(inode, pgoff);
+	/* Our caller expects us to return -EFAULT if we failed to find folio */
+	if (IS_ERR_OR_NULL(folio))
+		return -EFAULT;
+
+	page = folio_file_page(folio, pgoff);
+	if (PageHWPoison(page)) {
+		ret = -EIO;
+		goto out_release;
+	}
+
+	ret = mfill_atomic_install_pte(dst_pmd, dst_vma, dst_addr,
+				       page, flags);
+	if (ret)
+		goto out_release;
+
+	return 0;
+
+out_release:
+	folio_unlock(folio);
+	folio_put(folio);
+	return ret;
+}
+
+/* Handles UFFDIO_POISON for all non-hugetlb VMAs. */
+static int mfill_atomic_pte_poison(struct mfill_state *state)
+{
+	struct vm_area_struct *dst_vma = state->vma;
+	struct mm_struct *dst_mm = dst_vma->vm_mm;
+	unsigned long dst_addr = state->dst_addr;
+	pmd_t *dst_pmd = state->pmd;
+	pte_t _dst_pte, *dst_pte;
+	spinlock_t *ptl;
+	int ret;
+
+	_dst_pte = make_pte_marker(PTE_MARKER_POISONED);
+	ret = -EAGAIN;
+	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
+	if (!dst_pte)
+		goto out;
+
+	if (mfill_file_over_size(dst_vma, dst_addr)) {
+		ret = -EFAULT;
+		goto out_unlock;
+	}
+
+	ret = -EEXIST;
+	/* Refuse to overwrite any PTE, even a PTE marker (e.g. UFFD WP). */
+	if (!pte_none(ptep_get(dst_pte)))
+		goto out_unlock;
+
+	set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
+
+	/* No need to invalidate - it was non-present before */
+	update_mmu_cache(dst_vma, dst_addr, dst_pte);
+	ret = 0;
+out_unlock:
+	pte_unmap_unlock(dst_pte, ptl);
+out:
+	return ret;
+}
+
+#ifdef CONFIG_HUGETLB_PAGE
+/*
+ * mfill_atomic processing for HUGETLB vmas.  Note that this routine is
+ * called with either vma-lock or mmap_lock held, it will release the lock
+ * before returning.
+ */
+static __always_inline ssize_t mfill_atomic_hugetlb(
+					      struct userfaultfd_ctx *ctx,
+					      struct vm_area_struct *dst_vma,
+					      unsigned long dst_start,
+					      unsigned long src_start,
+					      unsigned long len,
+					      uffd_flags_t flags)
+{
+	struct mm_struct *dst_mm = dst_vma->vm_mm;
+	ssize_t err;
+	pte_t *dst_pte;
+	unsigned long src_addr, dst_addr;
+	long copied;
+	struct folio *folio;
+	unsigned long vma_hpagesize;
+	pgoff_t idx;
+	u32 hash;
+	struct address_space *mapping;
+
+	/*
+	 * There is no default zero huge page for all huge page sizes as
+	 * supported by hugetlb.  A PMD_SIZE huge pages may exist as used
+	 * by THP.  Since we can not reliably insert a zero page, this
+	 * feature is not supported.
+	 */
+	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE)) {
+		up_read(&ctx->map_changing_lock);
+		uffd_mfill_unlock(dst_vma);
+		return -EINVAL;
+	}
+
+	src_addr = src_start;
+	dst_addr = dst_start;
+	copied = 0;
+	folio = NULL;
+	vma_hpagesize = vma_kernel_pagesize(dst_vma);
+
+	/*
+	 * Validate alignment based on huge page size
+	 */
+	err = -EINVAL;
+	if (dst_start & (vma_hpagesize - 1) || len & (vma_hpagesize - 1))
+		goto out_unlock;
+
+retry:
+	/*
+	 * On routine entry dst_vma is set.  If we had to drop mmap_lock and
+	 * retry, dst_vma will be set to NULL and we must lookup again.
+	 */
+	if (!dst_vma) {
+		dst_vma = uffd_mfill_lock(dst_mm, dst_start, len);
+		if (IS_ERR(dst_vma)) {
+			err = PTR_ERR(dst_vma);
+			goto out;
+		}
+
+		err = -ENOENT;
+		if (!is_vm_hugetlb_page(dst_vma))
+			goto out_unlock_vma;
+
+		err = -EINVAL;
+		if (vma_hpagesize != vma_kernel_pagesize(dst_vma))
+			goto out_unlock_vma;
+
+		/*
+		 * If memory mappings are changing because of non-cooperative
+		 * operation (e.g. mremap) running in parallel, bail out and
+		 * request the user to retry later
+		 */
+		down_read(&ctx->map_changing_lock);
+		err = -EAGAIN;
+		if (atomic_read(&ctx->mmap_changing))
+			goto out_unlock;
+	}
+
+	while (src_addr < src_start + len) {
+		VM_WARN_ON_ONCE(dst_addr >= dst_start + len);
+
+		/*
+		 * Serialize via vma_lock and hugetlb_fault_mutex.
+		 * vma_lock ensures the dst_pte remains valid even
+		 * in the case of shared pmds.  fault mutex prevents
+		 * races with other faulting threads.
+		 */
+		idx = hugetlb_linear_page_index(dst_vma, dst_addr);
+		mapping = dst_vma->vm_file->f_mapping;
+		hash = hugetlb_fault_mutex_hash(mapping, idx);
+		mutex_lock(&hugetlb_fault_mutex_table[hash]);
+		hugetlb_vma_lock_read(dst_vma);
+
+		err = -ENOMEM;
+		dst_pte = huge_pte_alloc(dst_mm, dst_vma, dst_addr, vma_hpagesize);
+		if (!dst_pte) {
+			hugetlb_vma_unlock_read(dst_vma);
+			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+			goto out_unlock;
+		}
+
+		if (!uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE)) {
+			const pte_t ptep = huge_ptep_get(dst_mm, dst_addr, dst_pte);
+
+			if (!huge_pte_none(ptep) && !pte_is_uffd_marker(ptep)) {
+				err = -EEXIST;
+				hugetlb_vma_unlock_read(dst_vma);
+				mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+				goto out_unlock;
+			}
+		}
+
+		err = hugetlb_mfill_atomic_pte(dst_pte, dst_vma, dst_addr,
+					       src_addr, flags, &folio);
+
+		hugetlb_vma_unlock_read(dst_vma);
+		mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+
+		cond_resched();
+
+		if (unlikely(err == -ENOENT)) {
+			up_read(&ctx->map_changing_lock);
+			uffd_mfill_unlock(dst_vma);
+			VM_WARN_ON_ONCE(!folio);
+
+			err = copy_folio_from_user(folio,
+						   (const void __user *)src_addr, true);
+			if (unlikely(err)) {
+				err = -EFAULT;
+				goto out;
+			}
+
+			dst_vma = NULL;
+			goto retry;
+		} else
+			VM_WARN_ON_ONCE(folio);
+
+		if (!err) {
+			dst_addr += vma_hpagesize;
+			src_addr += vma_hpagesize;
+			copied += vma_hpagesize;
+
+			if (fatal_signal_pending(current))
+				err = -EINTR;
+		}
+		if (err)
+			break;
+	}
+
+out_unlock:
+	up_read(&ctx->map_changing_lock);
+out_unlock_vma:
+	uffd_mfill_unlock(dst_vma);
+out:
+	if (folio)
+		folio_put(folio);
+	VM_WARN_ON_ONCE(copied < 0);
+	VM_WARN_ON_ONCE(err > 0);
+	VM_WARN_ON_ONCE(!copied && !err);
+	return copied ? copied : err;
+}
+#else /* !CONFIG_HUGETLB_PAGE */
+/* fail at build time if gcc attempts to use this */
+extern ssize_t mfill_atomic_hugetlb(struct userfaultfd_ctx *ctx,
+				    struct vm_area_struct *dst_vma,
+				    unsigned long dst_start,
+				    unsigned long src_start,
+				    unsigned long len,
+				    uffd_flags_t flags);
+#endif /* CONFIG_HUGETLB_PAGE */
+
+static __always_inline ssize_t mfill_atomic_pte(struct mfill_state *state)
+{
+	uffd_flags_t flags = state->flags;
+
+	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
+		return mfill_atomic_pte_continue(state);
+	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_POISON))
+		return mfill_atomic_pte_poison(state);
+	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_COPY))
+		return mfill_atomic_pte_copy(state);
+	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE))
+		return mfill_atomic_pte_zeropage(state);
+
+	VM_WARN_ONCE(1, "Unknown UFFDIO operation, flags: %x", flags);
+	return -EOPNOTSUPP;
+}
+
+static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
+					    unsigned long dst_start,
+					    unsigned long src_start,
+					    unsigned long len,
+					    uffd_flags_t flags)
+{
+	struct mfill_state state = (struct mfill_state){
+		.ctx = ctx,
+		.dst_start = dst_start,
+		.src_start = src_start,
+		.flags = flags,
+		.len = len,
+		.src_addr = src_start,
+		.dst_addr = dst_start,
+	};
+	long copied = 0;
+	ssize_t err;
+
+	/*
+	 * Sanitize the command parameters:
+	 */
+	VM_WARN_ON_ONCE(dst_start & ~PAGE_MASK);
+	VM_WARN_ON_ONCE(len & ~PAGE_MASK);
+
+	/* Does the address range wrap, or is the span zero-sized? */
+	VM_WARN_ON_ONCE(src_start + len <= src_start);
+	VM_WARN_ON_ONCE(dst_start + len <= dst_start);
+
+	err = mfill_get_vma(&state);
+	if (err)
+		goto out;
+
+	/*
+	 * If this is a HUGETLB vma, pass off to appropriate routine
+	 */
+	if (is_vm_hugetlb_page(state.vma))
+		return  mfill_atomic_hugetlb(ctx, state.vma, dst_start,
+					     src_start, len, flags);
+
+	while (state.src_addr < src_start + len) {
+		VM_WARN_ON_ONCE(state.dst_addr >= dst_start + len);
+
+		err = mfill_establish_pmd(&state);
+		if (err)
+			break;
+
+		/*
+		 * For shmem mappings, khugepaged is allowed to remove page
+		 * tables under us; pte_offset_map_lock() will deal with that.
+		 */
+
+		err = mfill_atomic_pte(&state);
+		cond_resched();
+
+		if (!err) {
+			state.dst_addr += PAGE_SIZE;
+			state.src_addr += PAGE_SIZE;
+			copied += PAGE_SIZE;
+
+			if (fatal_signal_pending(current))
+				err = -EINTR;
+		}
+		if (err)
+			break;
+	}
+
+	mfill_put_vma(&state);
+out:
+	VM_WARN_ON_ONCE(copied < 0);
+	VM_WARN_ON_ONCE(err > 0);
+	VM_WARN_ON_ONCE(!copied && !err);
+	return copied ? copied : err;
+}
+
+static ssize_t mfill_atomic_copy(struct userfaultfd_ctx *ctx, unsigned long dst_start,
+			  unsigned long src_start, unsigned long len,
+			  uffd_flags_t flags)
+{
+	return mfill_atomic(ctx, dst_start, src_start, len,
+			    uffd_flags_set_mode(flags, MFILL_ATOMIC_COPY));
+}
+
+static ssize_t mfill_atomic_zeropage(struct userfaultfd_ctx *ctx,
+			      unsigned long start,
+			      unsigned long len)
+{
+	return mfill_atomic(ctx, start, 0, len,
+			    uffd_flags_set_mode(0, MFILL_ATOMIC_ZEROPAGE));
+}
+
+static ssize_t mfill_atomic_continue(struct userfaultfd_ctx *ctx, unsigned long start,
+			      unsigned long len, uffd_flags_t flags)
+{
+
+	/*
+	 * A caller might reasonably assume that UFFDIO_CONTINUE contains an
+	 * smp_wmb() to ensure that any writes to the about-to-be-mapped page by
+	 * the thread doing the UFFDIO_CONTINUE are guaranteed to be visible to
+	 * subsequent loads from the page through the newly mapped address range.
+	 */
+	smp_wmb();
+
+	return mfill_atomic(ctx, start, 0, len,
+			    uffd_flags_set_mode(flags, MFILL_ATOMIC_CONTINUE));
+}
+
+static ssize_t mfill_atomic_poison(struct userfaultfd_ctx *ctx, unsigned long start,
+			    unsigned long len, uffd_flags_t flags)
+{
+	return mfill_atomic(ctx, start, 0, len,
+			    uffd_flags_set_mode(flags, MFILL_ATOMIC_POISON));
+}
+
+long uffd_wp_range(struct vm_area_struct *dst_vma,
+		   unsigned long start, unsigned long len, bool enable_wp)
+{
+	unsigned int mm_cp_flags;
+	struct mmu_gather tlb;
+	long ret;
+
+	VM_WARN_ONCE(start < dst_vma->vm_start || start + len > dst_vma->vm_end,
+			"The address range exceeds VMA boundary.\n");
+	if (enable_wp)
+		mm_cp_flags = MM_CP_UFFD_WP;
+	else
+		mm_cp_flags = MM_CP_UFFD_WP_RESOLVE;
+
+	/*
+	 * vma->vm_page_prot already reflects that uffd-wp is enabled for this
+	 * VMA (see userfaultfd_set_vm_flags()) and that all PTEs are supposed
+	 * to be write-protected as default whenever protection changes.
+	 * Try upgrading write permissions manually.
+	 */
+	if (!enable_wp && vma_wants_manual_pte_write_upgrade(dst_vma))
+		mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;
+	tlb_gather_mmu(&tlb, dst_vma->vm_mm);
+	ret = change_protection(&tlb, dst_vma, start, start + len, mm_cp_flags);
+	tlb_finish_mmu(&tlb);
+
+	return ret;
+}
+
+static int mwriteprotect_range(struct userfaultfd_ctx *ctx, unsigned long start,
+			unsigned long len, bool enable_wp)
+{
+	struct mm_struct *dst_mm = ctx->mm;
+	unsigned long end = start + len;
+	unsigned long _start, _end;
+	struct vm_area_struct *dst_vma;
+	unsigned long page_mask;
+	long err;
+	VMA_ITERATOR(vmi, dst_mm, start);
+
+	/*
+	 * Sanitize the command parameters:
+	 */
+	VM_WARN_ON_ONCE(start & ~PAGE_MASK);
+	VM_WARN_ON_ONCE(len & ~PAGE_MASK);
+
+	/* Does the address range wrap, or is the span zero-sized? */
+	VM_WARN_ON_ONCE(start + len <= start);
+
+	mmap_read_lock(dst_mm);
+
+	/*
+	 * If memory mappings are changing because of non-cooperative
+	 * operation (e.g. mremap) running in parallel, bail out and
+	 * request the user to retry later
+	 */
+	down_read(&ctx->map_changing_lock);
+	err = -EAGAIN;
+	if (atomic_read(&ctx->mmap_changing))
+		goto out_unlock;
+
+	err = -ENOENT;
+	for_each_vma_range(vmi, dst_vma, end) {
+
+		if (!userfaultfd_wp(dst_vma)) {
+			err = -ENOENT;
+			break;
+		}
+
+		if (is_vm_hugetlb_page(dst_vma)) {
+			err = -EINVAL;
+			page_mask = vma_kernel_pagesize(dst_vma) - 1;
+			if ((start & page_mask) || (len & page_mask))
+				break;
+		}
+
+		_start = max(dst_vma->vm_start, start);
+		_end = min(dst_vma->vm_end, end);
+
+		err = uffd_wp_range(dst_vma, _start, _end - _start, enable_wp);
+
+		/* Return 0 on success, <0 on failures */
+		if (err < 0)
+			break;
+		err = 0;
+	}
+out_unlock:
+	up_read(&ctx->map_changing_lock);
+	mmap_read_unlock(dst_mm);
+	return err;
+}
+
+int mrwprotect_range(struct userfaultfd_ctx *ctx, unsigned long start,
+		     unsigned long len, bool enable_rwp)
+{
+	struct mm_struct *dst_mm = ctx->mm;
+	unsigned long end = start + len;
+	struct vm_area_struct *dst_vma;
+	unsigned int mm_cp_flags;
+	struct mmu_gather tlb;
+	bool found = false;
+	VMA_ITERATOR(vmi, dst_mm, start);
+
+	VM_WARN_ON_ONCE(start & ~PAGE_MASK);
+	VM_WARN_ON_ONCE(len & ~PAGE_MASK);
+	VM_WARN_ON_ONCE(start + len <= start);
+
+	guard(mmap_read_lock)(dst_mm);
+	guard(rwsem_read)(&ctx->map_changing_lock);
+
+	if (atomic_read(&ctx->mmap_changing))
+		return -EAGAIN;
+
+	if (enable_rwp)
+		mm_cp_flags = MM_CP_UFFD_RWP;
+	else
+		mm_cp_flags = MM_CP_UFFD_RWP_RESOLVE;
+
+	/*
+	 * Pre-scan the range: validate every spanned VMA before applying
+	 * any change_protection() so a partial failure cannot leave the
+	 * process with only a prefix of the range re-protected.
+	 */
+	for_each_vma_range(vmi, dst_vma, end) {
+		if (!userfaultfd_rwp(dst_vma))
+			return -ENOENT;
+
+		if (is_vm_hugetlb_page(dst_vma)) {
+			unsigned long page_mask;
+
+			page_mask = vma_kernel_pagesize(dst_vma) - 1;
+			if ((start & page_mask) || (len & page_mask))
+				return -EINVAL;
+		}
+		found = true;
+	}
+	if (!found)
+		return -ENOENT;
+
+	vma_iter_set(&vmi, start);
+	tlb_gather_mmu(&tlb, dst_mm);
+	for_each_vma_range(vmi, dst_vma, end) {
+		unsigned long vma_start = max(dst_vma->vm_start, start);
+		unsigned long vma_end = min(dst_vma->vm_end, end);
+		unsigned int flags = mm_cp_flags;
+
+		/*
+		 * On resolve, try to upgrade writability per-VMA --
+		 * MM_CP_TRY_CHANGE_WRITABLE WARNs in
+		 * maybe_change_pte_writable() if the VMA is not VM_WRITE,
+		 * and RWP can be registered on PROT_READ-only mappings.
+		 */
+		if (!enable_rwp && vma_wants_manual_pte_write_upgrade(dst_vma))
+			flags |= MM_CP_TRY_CHANGE_WRITABLE;
+
+		change_protection(&tlb, dst_vma, vma_start, vma_end, flags);
+	}
+	tlb_finish_mmu(&tlb);
+
+	return 0;
+}
+
+void double_pt_lock(spinlock_t *ptl1,
+		    spinlock_t *ptl2)
+	__acquires(ptl1)
+	__acquires(ptl2)
+{
+	if (ptl1 > ptl2)
+		swap(ptl1, ptl2);
+	/* lock in virtual address order to avoid lock inversion */
+	spin_lock(ptl1);
+	if (ptl1 != ptl2)
+		spin_lock_nested(ptl2, SINGLE_DEPTH_NESTING);
+	else
+		__acquire(ptl2);
+}
+
+void double_pt_unlock(spinlock_t *ptl1,
+		      spinlock_t *ptl2)
+	__releases(ptl1)
+	__releases(ptl2)
+{
+	spin_unlock(ptl1);
+	if (ptl1 != ptl2)
+		spin_unlock(ptl2);
+	else
+		__release(ptl2);
+}
+
+static inline bool is_pte_pages_stable(pte_t *dst_pte, pte_t *src_pte,
+				       pte_t orig_dst_pte, pte_t orig_src_pte,
+				       pmd_t *dst_pmd, pmd_t dst_pmdval)
+{
+	return pte_same(ptep_get(src_pte), orig_src_pte) &&
+	       pte_same(ptep_get(dst_pte), orig_dst_pte) &&
+	       pmd_same(dst_pmdval, pmdp_get_lockless(dst_pmd));
+}
+
+/*
+ * Checks if the two ptes and the corresponding folio are eligible for batched
+ * move. If so, then returns pointer to the locked folio. Otherwise, returns NULL.
+ *
+ * NOTE: folio's reference is not required as the whole operation is within
+ * PTL's critical section.
+ */
+static struct folio *check_ptes_for_batched_move(struct vm_area_struct *src_vma,
+						 unsigned long src_addr,
+						 pte_t *src_pte, pte_t *dst_pte)
+{
+	pte_t orig_dst_pte, orig_src_pte;
+	struct folio *folio;
+
+	orig_dst_pte = ptep_get(dst_pte);
+	if (!pte_none(orig_dst_pte))
+		return NULL;
+
+	orig_src_pte = ptep_get(src_pte);
+	if (!pte_present(orig_src_pte) || is_zero_pfn(pte_pfn(orig_src_pte)))
+		return NULL;
+
+	folio = vm_normal_folio(src_vma, src_addr, orig_src_pte);
+	if (!folio || !folio_trylock(folio))
+		return NULL;
+	if (!PageAnonExclusive(&folio->page) || folio_test_large(folio)) {
+		folio_unlock(folio);
+		return NULL;
+	}
+	return folio;
+}
+
+/*
+ * Moves src folios to dst in a batch as long as they are not large, and can
+ * successfully take the lock via folio_trylock().
+ */
+static long move_present_ptes(struct mm_struct *mm,
+			      struct vm_area_struct *dst_vma,
+			      struct vm_area_struct *src_vma,
+			      unsigned long dst_addr, unsigned long src_addr,
+			      pte_t *dst_pte, pte_t *src_pte,
+			      pte_t orig_dst_pte, pte_t orig_src_pte,
+			      pmd_t *dst_pmd, pmd_t dst_pmdval,
+			      spinlock_t *dst_ptl, spinlock_t *src_ptl,
+			      struct folio **first_src_folio, unsigned long len)
+{
+	int err = 0;
+	struct folio *src_folio = *first_src_folio;
+	unsigned long src_start = src_addr;
+	unsigned long src_end;
+
+	len = pmd_addr_end(dst_addr, dst_addr + len) - dst_addr;
+	src_end = pmd_addr_end(src_addr, src_addr + len);
+	flush_cache_range(src_vma, src_addr, src_end);
+	double_pt_lock(dst_ptl, src_ptl);
+
+	if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte,
+				 dst_pmd, dst_pmdval)) {
+		err = -EAGAIN;
+		goto out;
+	}
+	if (folio_test_large(src_folio) ||
+	    folio_maybe_dma_pinned(src_folio) ||
+	    !PageAnonExclusive(&src_folio->page)) {
+		err = -EBUSY;
+		goto out;
+	}
+	/* It's safe to drop the reference now as the page-table is holding one. */
+	folio_put(*first_src_folio);
+	*first_src_folio = NULL;
+	lazy_mmu_mode_enable();
+
+	while (true) {
+		orig_src_pte = ptep_get_and_clear(mm, src_addr, src_pte);
+		/* Folio got pinned from under us. Put it back and fail the move. */
+		if (folio_maybe_dma_pinned(src_folio)) {
+			set_pte_at(mm, src_addr, src_pte, orig_src_pte);
+			err = -EBUSY;
+			break;
+		}
+
+		folio_move_anon_rmap(src_folio, dst_vma);
+		src_folio->index = linear_page_index(dst_vma, dst_addr);
+
+		orig_dst_pte = folio_mk_pte(src_folio, dst_vma->vm_page_prot);
+		/* Set soft dirty bit so userspace can notice the pte was moved */
+		if (pgtable_supports_soft_dirty())
+			orig_dst_pte = pte_mksoft_dirty(orig_dst_pte);
+		if (pte_dirty(orig_src_pte))
+			orig_dst_pte = pte_mkdirty(orig_dst_pte);
+		orig_dst_pte = pte_mkwrite(orig_dst_pte, dst_vma);
+
+		/* Re-arm RWP on the moved PTE if dst_vma is RWP-registered. */
+		if (userfaultfd_rwp(dst_vma)) {
+			orig_dst_pte = pte_modify(orig_dst_pte, PAGE_NONE);
+			orig_dst_pte = pte_mkuffd(orig_dst_pte);
+		}
+
+		set_pte_at(mm, dst_addr, dst_pte, orig_dst_pte);
+
+		src_addr += PAGE_SIZE;
+		if (src_addr == src_end)
+			break;
+		dst_addr += PAGE_SIZE;
+		dst_pte++;
+		src_pte++;
+
+		folio_unlock(src_folio);
+		src_folio = check_ptes_for_batched_move(src_vma, src_addr,
+							src_pte, dst_pte);
+		if (!src_folio)
+			break;
+	}
+
+	lazy_mmu_mode_disable();
+	if (src_addr > src_start)
+		flush_tlb_range(src_vma, src_start, src_addr);
+
+	if (src_folio)
+		folio_unlock(src_folio);
+out:
+	double_pt_unlock(dst_ptl, src_ptl);
+	return src_addr > src_start ? src_addr - src_start : err;
+}
+
+static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma,
+			 unsigned long dst_addr, unsigned long src_addr,
+			 pte_t *dst_pte, pte_t *src_pte,
+			 pte_t orig_dst_pte, pte_t orig_src_pte,
+			 pmd_t *dst_pmd, pmd_t dst_pmdval,
+			 spinlock_t *dst_ptl, spinlock_t *src_ptl,
+			 struct folio *src_folio,
+			 struct swap_info_struct *si, swp_entry_t entry)
+{
+	/*
+	 * Check if the folio still belongs to the target swap entry after
+	 * acquiring the lock. Folio can be freed in the swap cache while
+	 * not locked.
+	 */
+	if (src_folio && unlikely(!folio_test_swapcache(src_folio) ||
+				  entry.val != src_folio->swap.val))
+		return -EAGAIN;
+
+	double_pt_lock(dst_ptl, src_ptl);
+
+	if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte,
+				 dst_pmd, dst_pmdval)) {
+		double_pt_unlock(dst_ptl, src_ptl);
+		return -EAGAIN;
+	}
+
+	/*
+	 * The src_folio resides in the swapcache, requiring an update to its
+	 * index and mapping to align with the dst_vma, where a swap-in may
+	 * occur and hit the swapcache after moving the PTE.
+	 */
+	if (src_folio) {
+		folio_move_anon_rmap(src_folio, dst_vma);
+		src_folio->index = linear_page_index(dst_vma, dst_addr);
+	} else {
+		/*
+		 * Check if the swap entry is cached after acquiring the src_pte
+		 * lock. Otherwise, we might miss a newly loaded swap cache folio.
+		 *
+		 * We are trying to catch newly added swap cache, the only possible case is
+		 * when a folio is swapped in and out again staying in swap cache, using the
+		 * same entry before the PTE check above. The PTL is acquired and released
+		 * twice, each time after updating the swap table. So holding
+		 * the PTL here ensures we see the updated value.
+		 */
+		if (swap_cache_has_folio(entry)) {
+			double_pt_unlock(dst_ptl, src_ptl);
+			return -EAGAIN;
+		}
+	}
+
+	orig_src_pte = ptep_get_and_clear(mm, src_addr, src_pte);
+	if (pgtable_supports_soft_dirty())
+		orig_src_pte = pte_swp_mksoft_dirty(orig_src_pte);
+	/* Re-arm RWP on the moved swap entry if dst_vma is RWP-registered. */
+	if (userfaultfd_rwp(dst_vma))
+		orig_src_pte = pte_swp_mkuffd(orig_src_pte);
+	set_pte_at(mm, dst_addr, dst_pte, orig_src_pte);
+	double_pt_unlock(dst_ptl, src_ptl);
+
+	return PAGE_SIZE;
+}
+
+static int move_zeropage_pte(struct mm_struct *mm,
+			     struct vm_area_struct *dst_vma,
+			     struct vm_area_struct *src_vma,
+			     unsigned long dst_addr, unsigned long src_addr,
+			     pte_t *dst_pte, pte_t *src_pte,
+			     pte_t orig_dst_pte, pte_t orig_src_pte,
+			     pmd_t *dst_pmd, pmd_t dst_pmdval,
+			     spinlock_t *dst_ptl, spinlock_t *src_ptl)
+{
+	pte_t zero_pte;
+
+	double_pt_lock(dst_ptl, src_ptl);
+	if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte,
+				 dst_pmd, dst_pmdval)) {
+		double_pt_unlock(dst_ptl, src_ptl);
+		return -EAGAIN;
+	}
+
+	zero_pte = pte_mkspecial(pfn_pte(zero_pfn(dst_addr),
+					 dst_vma->vm_page_prot));
+
+	/* Re-arm RWP on the moved PTE if dst_vma is RWP-registered. */
+	if (userfaultfd_rwp(dst_vma)) {
+		zero_pte = pte_modify(zero_pte, PAGE_NONE);
+		zero_pte = pte_mkuffd(zero_pte);
+	}
+
+	ptep_clear_flush(src_vma, src_addr, src_pte);
+	set_pte_at(mm, dst_addr, dst_pte, zero_pte);
+	double_pt_unlock(dst_ptl, src_ptl);
+
+	return PAGE_SIZE;
+}
+
+
+/*
+ * The mmap_lock for reading is held by the caller. Just move the page(s)
+ * from src_pmd to dst_pmd if possible, and return number of bytes moved.
+ * On failure, an error code is returned.
+ */
+static long move_pages_ptes(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
+			    struct vm_area_struct *dst_vma,
+			    struct vm_area_struct *src_vma,
+			    unsigned long dst_addr, unsigned long src_addr,
+			    unsigned long len, __u64 mode)
+{
+	struct swap_info_struct *si = NULL;
+	pte_t orig_src_pte, orig_dst_pte;
+	pte_t src_folio_pte;
+	spinlock_t *src_ptl, *dst_ptl;
+	pte_t *src_pte = NULL;
+	pte_t *dst_pte = NULL;
+	pmd_t dummy_pmdval;
+	pmd_t dst_pmdval;
+	struct folio *src_folio = NULL;
+	struct mmu_notifier_range range;
+	long ret = 0;
+
+	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm,
+				src_addr, src_addr + len);
+	mmu_notifier_invalidate_range_start(&range);
+retry:
+	/*
+	 * Use the maywrite version to indicate that dst_pte will be modified,
+	 * since dst_pte needs to be none, the subsequent pte_same() check
+	 * cannot prevent the dst_pte page from being freed concurrently, so we
+	 * also need to obtain dst_pmdval and recheck pmd_same() later.
+	 */
+	dst_pte = pte_offset_map_rw_nolock(mm, dst_pmd, dst_addr, &dst_pmdval,
+					   &dst_ptl);
+
+	/* Retry if a huge pmd materialized from under us */
+	if (unlikely(!dst_pte)) {
+		ret = -EAGAIN;
+		goto out;
+	}
+
+	/*
+	 * Unlike dst_pte, the subsequent pte_same() check can ensure the
+	 * stability of the src_pte page, so there is no need to get pmdval,
+	 * just pass a dummy variable to it.
+	 */
+	src_pte = pte_offset_map_rw_nolock(mm, src_pmd, src_addr, &dummy_pmdval,
+					   &src_ptl);
+
+	/*
+	 * We held the mmap_lock for reading so MADV_DONTNEED
+	 * can zap transparent huge pages under us, or the
+	 * transparent huge page fault can establish new
+	 * transparent huge pages under us.
+	 */
+	if (unlikely(!src_pte)) {
+		ret = -EAGAIN;
+		goto out;
+	}
+
+	/* Sanity checks before the operation */
+	if (pmd_none(*dst_pmd) || pmd_none(*src_pmd) ||
+	    pmd_trans_huge(*dst_pmd) || pmd_trans_huge(*src_pmd)) {
+		ret = -EINVAL;
+		goto out;
+	}
+
+	spin_lock(dst_ptl);
+	orig_dst_pte = ptep_get(dst_pte);
+	spin_unlock(dst_ptl);
+	if (!pte_none(orig_dst_pte)) {
+		ret = -EEXIST;
+		goto out;
+	}
+
+	spin_lock(src_ptl);
+	orig_src_pte = ptep_get(src_pte);
+	spin_unlock(src_ptl);
+	if (pte_none(orig_src_pte)) {
+		if (!(mode & UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES))
+			ret = -ENOENT;
+		else /* nothing to do to move a hole */
+			ret = PAGE_SIZE;
+		goto out;
+	}
+
+	/* If PTE changed after we locked the folio then start over */
+	if (src_folio && unlikely(!pte_same(src_folio_pte, orig_src_pte))) {
+		ret = -EAGAIN;
+		goto out;
+	}
+
+	if (pte_present(orig_src_pte)) {
+		if (is_zero_pfn(pte_pfn(orig_src_pte))) {
+			ret = move_zeropage_pte(mm, dst_vma, src_vma,
+					       dst_addr, src_addr, dst_pte, src_pte,
+					       orig_dst_pte, orig_src_pte,
+					       dst_pmd, dst_pmdval, dst_ptl, src_ptl);
+			goto out;
+		}
+
+		/*
+		 * Pin and lock source folio. Since we are in RCU read section,
+		 * we can't block, so on contention have to unmap the ptes,
+		 * obtain the lock and retry.
+		 */
+		if (!src_folio) {
+			struct folio *folio;
+			bool locked;
+
+			/*
+			 * Pin the page while holding the lock to be sure the
+			 * page isn't freed under us
+			 */
+			spin_lock(src_ptl);
+			if (!pte_same(orig_src_pte, ptep_get(src_pte))) {
+				spin_unlock(src_ptl);
+				ret = -EAGAIN;
+				goto out;
+			}
+
+			folio = vm_normal_folio(src_vma, src_addr, orig_src_pte);
+			if (!folio || !PageAnonExclusive(&folio->page)) {
+				spin_unlock(src_ptl);
+				ret = -EBUSY;
+				goto out;
+			}
+
+			locked = folio_trylock(folio);
+			/*
+			 * We avoid waiting for folio lock with a raised
+			 * refcount for large folios because extra refcounts
+			 * will result in split_folio() failing later and
+			 * retrying.  If multiple tasks are trying to move a
+			 * large folio we can end up livelocking.
+			 */
+			if (!locked && folio_test_large(folio)) {
+				spin_unlock(src_ptl);
+				ret = -EAGAIN;
+				goto out;
+			}
+
+			folio_get(folio);
+			src_folio = folio;
+			src_folio_pte = orig_src_pte;
+			spin_unlock(src_ptl);
+
+			if (!locked) {
+				pte_unmap(src_pte);
+				pte_unmap(dst_pte);
+				src_pte = dst_pte = NULL;
+				/* now we can block and wait */
+				folio_lock(src_folio);
+				goto retry;
+			}
+
+			if (WARN_ON_ONCE(!folio_test_anon(src_folio))) {
+				ret = -EBUSY;
+				goto out;
+			}
+		}
+
+		/* at this point we have src_folio locked */
+		if (folio_test_large(src_folio)) {
+			/* split_folio() can block */
+			pte_unmap(src_pte);
+			pte_unmap(dst_pte);
+			src_pte = dst_pte = NULL;
+			ret = split_folio(src_folio);
+			if (ret)
+				goto out;
+			/* have to reacquire the folio after it got split */
+			folio_unlock(src_folio);
+			folio_put(src_folio);
+			src_folio = NULL;
+			goto retry;
+		}
+
+		ret = move_present_ptes(mm, dst_vma, src_vma,
+					dst_addr, src_addr, dst_pte, src_pte,
+					orig_dst_pte, orig_src_pte, dst_pmd,
+					dst_pmdval, dst_ptl, src_ptl, &src_folio,
+					len);
+	} else { /* !pte_present() */
+		struct folio *folio = NULL;
+		const softleaf_t entry = softleaf_from_pte(orig_src_pte);
+
+		if (softleaf_is_migration(entry)) {
+			pte_unmap(src_pte);
+			pte_unmap(dst_pte);
+			src_pte = dst_pte = NULL;
+			migration_entry_wait(mm, src_pmd, src_addr);
+
+			ret = -EAGAIN;
+			goto out;
+		} else if (!softleaf_is_swap(entry)) {
+			ret = -EFAULT;
+			goto out;
+		}
+
+		if (!pte_swp_exclusive(orig_src_pte)) {
+			ret = -EBUSY;
+			goto out;
+		}
+
+		si = get_swap_device(entry);
+		if (unlikely(!si)) {
+			ret = -EAGAIN;
+			goto out;
+		}
+		/*
+		 * Verify the existence of the swapcache. If present, the folio's
+		 * index and mapping must be updated even when the PTE is a swap
+		 * entry. The anon_vma lock is not taken during this process since
+		 * the folio has already been unmapped, and the swap entry is
+		 * exclusive, preventing rmap walks.
+		 *
+		 * For large folios, return -EBUSY immediately, as split_folio()
+		 * also returns -EBUSY when attempting to split unmapped large
+		 * folios in the swapcache. This issue needs to be resolved
+		 * separately to allow proper handling.
+		 */
+		if (!src_folio)
+			folio = swap_cache_get_folio(entry);
+		if (folio) {
+			if (folio_test_large(folio)) {
+				ret = -EBUSY;
+				folio_put(folio);
+				goto out;
+			}
+			src_folio = folio;
+			src_folio_pte = orig_src_pte;
+			if (!folio_trylock(src_folio)) {
+				pte_unmap(src_pte);
+				pte_unmap(dst_pte);
+				src_pte = dst_pte = NULL;
+				put_swap_device(si);
+				si = NULL;
+				/* now we can block and wait */
+				folio_lock(src_folio);
+				goto retry;
+			}
+		}
+		ret = move_swap_pte(mm, dst_vma, dst_addr, src_addr, dst_pte, src_pte,
+				orig_dst_pte, orig_src_pte, dst_pmd, dst_pmdval,
+				dst_ptl, src_ptl, src_folio, si, entry);
+	}
+
+out:
+	if (src_folio) {
+		folio_unlock(src_folio);
+		folio_put(src_folio);
+	}
+	/*
+	 * Unmap in reverse order (LIFO) to maintain proper kmap_local
+	 * index ordering when CONFIG_HIGHPTE is enabled. We mapped dst_pte
+	 * first, then src_pte, so we must unmap src_pte first, then dst_pte.
+	 */
+	if (src_pte)
+		pte_unmap(src_pte);
+	if (dst_pte)
+		pte_unmap(dst_pte);
+	mmu_notifier_invalidate_range_end(&range);
+	if (si)
+		put_swap_device(si);
+
+	return ret;
+}
+
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+static inline bool move_splits_huge_pmd(unsigned long dst_addr,
+					unsigned long src_addr,
+					unsigned long src_end)
+{
+	return (src_addr & ~HPAGE_PMD_MASK) || (dst_addr & ~HPAGE_PMD_MASK) ||
+		src_end - src_addr < HPAGE_PMD_SIZE;
+}
+#else
+static inline bool move_splits_huge_pmd(unsigned long dst_addr,
+					unsigned long src_addr,
+					unsigned long src_end)
+{
+	/* This is unreachable anyway, just to avoid warnings when HPAGE_PMD_SIZE==0 */
+	return false;
+}
+#endif
+
+static inline bool vma_move_compatible(struct vm_area_struct *vma)
+{
+	return !(vma->vm_flags & (VM_PFNMAP | VM_IO |  VM_HUGETLB |
+				  VM_MIXEDMAP | VM_SHADOW_STACK));
+}
+
+static int validate_move_areas(struct userfaultfd_ctx *ctx,
+			       struct vm_area_struct *src_vma,
+			       struct vm_area_struct *dst_vma)
+{
+	/* Only allow moving if both have the same access and protection */
+	if ((src_vma->vm_flags & VM_ACCESS_FLAGS) != (dst_vma->vm_flags & VM_ACCESS_FLAGS) ||
+	    pgprot_val(src_vma->vm_page_prot) != pgprot_val(dst_vma->vm_page_prot))
+		return -EINVAL;
+
+	/* Only allow moving if both are mlocked or both aren't */
+	if ((src_vma->vm_flags & VM_LOCKED) != (dst_vma->vm_flags & VM_LOCKED))
+		return -EINVAL;
+
+	/*
+	 * For now, we keep it simple and only move between writable VMAs.
+	 * Access flags are equal, therefore checking only the source is enough.
+	 */
+	if (!(src_vma->vm_flags & VM_WRITE))
+		return -EINVAL;
+
+	/* Check if vma flags indicate content which can be moved */
+	if (!vma_move_compatible(src_vma) || !vma_move_compatible(dst_vma))
+		return -EINVAL;
+
+	/* Ensure dst_vma is registered in uffd we are operating on */
+	if (!dst_vma->vm_userfaultfd_ctx.ctx ||
+	    dst_vma->vm_userfaultfd_ctx.ctx != ctx)
+		return -EINVAL;
+
+	/* Only allow moving across anonymous vmas */
+	if (!vma_is_anonymous(src_vma) || !vma_is_anonymous(dst_vma))
+		return -EINVAL;
+
+	return 0;
+}
+
+static __always_inline
+int find_vmas_mm_locked(struct mm_struct *mm,
+			unsigned long dst_start,
+			unsigned long src_start,
+			struct vm_area_struct **dst_vmap,
+			struct vm_area_struct **src_vmap)
+{
+	struct vm_area_struct *vma;
+
+	mmap_assert_locked(mm);
+	vma = find_vma_and_prepare_anon(mm, dst_start);
+	if (IS_ERR(vma))
+		return PTR_ERR(vma);
+
+	*dst_vmap = vma;
+	/* Skip finding src_vma if src_start is in dst_vma */
+	if (src_start >= vma->vm_start && src_start < vma->vm_end)
+		goto out_success;
+
+	vma = vma_lookup(mm, src_start);
+	if (!vma)
+		return -ENOENT;
+out_success:
+	*src_vmap = vma;
+	return 0;
+}
+
+#ifdef CONFIG_PER_VMA_LOCK
+static int uffd_move_lock(struct mm_struct *mm,
+			  unsigned long dst_start,
+			  unsigned long src_start,
+			  struct vm_area_struct **dst_vmap,
+			  struct vm_area_struct **src_vmap)
+{
+	struct vm_area_struct *vma;
+	int err;
+
+	vma = uffd_lock_vma(mm, dst_start);
+	if (IS_ERR(vma))
+		return PTR_ERR(vma);
+
+	*dst_vmap = vma;
+	/*
+	 * Skip finding src_vma if src_start is in dst_vma. This also ensures
+	 * that we don't lock the same vma twice.
+	 */
+	if (src_start >= vma->vm_start && src_start < vma->vm_end) {
+		*src_vmap = vma;
+		return 0;
+	}
+
+	/*
+	 * Using uffd_lock_vma() to get src_vma can lead to following deadlock:
+	 *
+	 * Thread1				Thread2
+	 * -------				-------
+	 * vma_start_read(dst_vma)
+	 *					mmap_write_lock(mm)
+	 *					vma_start_write(src_vma)
+	 * vma_start_read(src_vma)
+	 * mmap_read_lock(mm)
+	 *					vma_start_write(dst_vma)
+	 */
+	*src_vmap = lock_vma_under_rcu(mm, src_start);
+	if (likely(*src_vmap))
+		return 0;
+
+	/* Undo any locking and retry in mmap_lock critical section */
+	vma_end_read(*dst_vmap);
+
+	mmap_read_lock(mm);
+	err = find_vmas_mm_locked(mm, dst_start, src_start, dst_vmap, src_vmap);
+	if (err)
+		goto out;
+
+	if (!vma_start_read_locked(*dst_vmap)) {
+		err = -EAGAIN;
+		goto out;
+	}
+
+	/* Nothing further to do if both vmas are locked. */
+	if (*dst_vmap == *src_vmap)
+		goto out;
+
+	if (!vma_start_read_locked_nested(*src_vmap, SINGLE_DEPTH_NESTING)) {
+		/* Undo dst_vmap locking if src_vmap failed to lock */
+		vma_end_read(*dst_vmap);
+		err = -EAGAIN;
+	}
+out:
+	mmap_read_unlock(mm);
+	return err;
+}
+
+static void uffd_move_unlock(struct vm_area_struct *dst_vma,
+			     struct vm_area_struct *src_vma)
+{
+	vma_end_read(src_vma);
+	if (src_vma != dst_vma)
+		vma_end_read(dst_vma);
+}
+
+#else
+
+static int uffd_move_lock(struct mm_struct *mm,
+			  unsigned long dst_start,
+			  unsigned long src_start,
+			  struct vm_area_struct **dst_vmap,
+			  struct vm_area_struct **src_vmap)
+{
+	int err;
+
+	mmap_read_lock(mm);
+	err = find_vmas_mm_locked(mm, dst_start, src_start, dst_vmap, src_vmap);
+	if (err)
+		mmap_read_unlock(mm);
+	return err;
+}
+
+static void uffd_move_unlock(struct vm_area_struct *dst_vma,
+			     struct vm_area_struct *src_vma)
+{
+	mmap_assert_locked(src_vma->vm_mm);
+	mmap_read_unlock(dst_vma->vm_mm);
+}
+#endif
+
+/**
+ * move_pages - move arbitrary anonymous pages of an existing vma
+ * @ctx: pointer to the userfaultfd context
+ * @dst_start: start of the destination virtual memory range
+ * @src_start: start of the source virtual memory range
+ * @len: length of the virtual memory range
+ * @mode: flags from uffdio_move.mode
+ *
+ * It will either use the mmap_lock in read mode or per-vma locks
+ *
+ * move_pages() remaps arbitrary anonymous pages atomically in zero
+ * copy. It only works on non shared anonymous pages because those can
+ * be relocated without generating non linear anon_vmas in the rmap
+ * code.
+ *
+ * It provides a zero copy mechanism to handle userspace page faults.
+ * The source vma pages should have mapcount == 1, which can be
+ * enforced by using madvise(MADV_DONTFORK) on src vma.
+ *
+ * The thread receiving the page during the userland page fault
+ * will receive the faulting page in the source vma through the network,
+ * storage or any other I/O device (MADV_DONTFORK in the source vma
+ * avoids move_pages() to fail with -EBUSY if the process forks before
+ * move_pages() is called), then it will call move_pages() to map the
+ * page in the faulting address in the destination vma.
+ *
+ * This userfaultfd command works purely via pagetables, so it's the
+ * most efficient way to move physical non shared anonymous pages
+ * across different virtual addresses. Unlike mremap()/mmap()/munmap()
+ * it does not create any new vmas. The mapping in the destination
+ * address is atomic.
+ *
+ * It only works if the vma protection bits are identical from the
+ * source and destination vma.
+ *
+ * It can remap non shared anonymous pages within the same vma too.
+ *
+ * If the source virtual memory range has any unmapped holes, or if
+ * the destination virtual memory range is not a whole unmapped hole,
+ * move_pages() will fail respectively with -ENOENT or -EEXIST. This
+ * provides a very strict behavior to avoid any chance of memory
+ * corruption going unnoticed if there are userland race conditions.
+ * Only one thread should resolve the userland page fault at any given
+ * time for any given faulting address. This means that if two threads
+ * try to both call move_pages() on the same destination address at the
+ * same time, the second thread will get an explicit error from this
+ * command.
+ *
+ * The command retval will return "len" is successful. The command
+ * however can be interrupted by fatal signals or errors. If
+ * interrupted it will return the number of bytes successfully
+ * remapped before the interruption if any, or the negative error if
+ * none. It will never return zero. Either it will return an error or
+ * an amount of bytes successfully moved. If the retval reports a
+ * "short" remap, the move_pages() command should be repeated by
+ * userland with src+retval, dst+reval, len-retval if it wants to know
+ * about the error that interrupted it.
+ *
+ * The UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES flag can be specified to
+ * prevent -ENOENT errors to materialize if there are holes in the
+ * source virtual range that is being remapped. The holes will be
+ * accounted as successfully remapped in the retval of the
+ * command. This is mostly useful to remap hugepage naturally aligned
+ * virtual regions without knowing if there are transparent hugepage
+ * in the regions or not, but preventing the risk of having to split
+ * the hugepmd during the remap.
+ */
+static ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
+		   unsigned long src_start, unsigned long len, __u64 mode)
+{
+	struct mm_struct *mm = ctx->mm;
+	struct vm_area_struct *src_vma, *dst_vma;
+	unsigned long src_addr, dst_addr, src_end;
+	pmd_t *src_pmd, *dst_pmd;
+	long err = -EINVAL;
+	ssize_t moved = 0;
+
+	/* Sanitize the command parameters. */
+	VM_WARN_ON_ONCE(src_start & ~PAGE_MASK);
+	VM_WARN_ON_ONCE(dst_start & ~PAGE_MASK);
+	VM_WARN_ON_ONCE(len & ~PAGE_MASK);
+
+	/* Does the address range wrap, or is the span zero-sized? */
+	VM_WARN_ON_ONCE(src_start + len < src_start);
+	VM_WARN_ON_ONCE(dst_start + len < dst_start);
+
+	err = uffd_move_lock(mm, dst_start, src_start, &dst_vma, &src_vma);
+	if (err)
+		goto out;
+
+	/* Re-check after taking map_changing_lock */
+	err = -EAGAIN;
+	down_read(&ctx->map_changing_lock);
+	if (likely(atomic_read(&ctx->mmap_changing)))
+		goto out_unlock;
+	/*
+	 * Make sure the vma is not shared, that the src and dst remap
+	 * ranges are both valid and fully within a single existing
+	 * vma.
+	 */
+	err = -EINVAL;
+	if (src_vma->vm_flags & VM_SHARED)
+		goto out_unlock;
+	if (src_start + len > src_vma->vm_end)
+		goto out_unlock;
+
+	if (dst_vma->vm_flags & VM_SHARED)
+		goto out_unlock;
+	if (dst_start + len > dst_vma->vm_end)
+		goto out_unlock;
+
+	err = validate_move_areas(ctx, src_vma, dst_vma);
+	if (err)
+		goto out_unlock;
+
+	for (src_addr = src_start, dst_addr = dst_start, src_end = src_start + len;
+	     src_addr < src_end;) {
+		spinlock_t *ptl;
+		pmd_t dst_pmdval;
+		unsigned long step_size;
+
+		/*
+		 * Below works because anonymous area would not have a
+		 * transparent huge PUD. If file-backed support is added,
+		 * that case would need to be handled here.
+		 */
+		src_pmd = mm_find_pmd(mm, src_addr);
+		if (unlikely(!src_pmd)) {
+			if (!(mode & UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES)) {
+				err = -ENOENT;
+				break;
+			}
+			src_pmd = mm_alloc_pmd(mm, src_addr);
+			if (unlikely(!src_pmd)) {
+				err = -ENOMEM;
+				break;
+			}
+		}
+		dst_pmd = mm_alloc_pmd(mm, dst_addr);
+		if (unlikely(!dst_pmd)) {
+			err = -ENOMEM;
+			break;
+		}
+
+		dst_pmdval = pmdp_get_lockless(dst_pmd);
+		/*
+		 * If the dst_pmd is mapped as THP don't override it and just
+		 * be strict. If dst_pmd changes into TPH after this check, the
+		 * move_pages_huge_pmd() will detect the change and retry
+		 * while move_pages_pte() will detect the change and fail.
+		 */
+		if (unlikely(pmd_trans_huge(dst_pmdval))) {
+			err = -EEXIST;
+			break;
+		}
+
+		ptl = pmd_trans_huge_lock(src_pmd, src_vma);
+		if (ptl) {
+			/* Check if we can move the pmd without splitting it. */
+			if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||
+			    !pmd_none(dst_pmdval)) {
+				/* Can be a migration entry */
+				if (pmd_present(*src_pmd)) {
+					struct folio *folio = pmd_folio(*src_pmd);
+
+					if (!is_huge_zero_folio(folio) &&
+					    !PageAnonExclusive(&folio->page)) {
+						spin_unlock(ptl);
+						err = -EBUSY;
+						break;
+					}
+				}
+
+				spin_unlock(ptl);
+				split_huge_pmd(src_vma, src_pmd, src_addr);
+				/* The folio will be split by move_pages_pte() */
+				continue;
+			}
+
+			err = move_pages_huge_pmd(mm, dst_pmd, src_pmd,
+						  dst_pmdval, dst_vma, src_vma,
+						  dst_addr, src_addr);
+			step_size = HPAGE_PMD_SIZE;
+		} else {
+			long ret;
+
+			if (pmd_none(*src_pmd)) {
+				if (!(mode & UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES)) {
+					err = -ENOENT;
+					break;
+				}
+				if (unlikely(__pte_alloc(mm, src_pmd))) {
+					err = -ENOMEM;
+					break;
+				}
+			}
+
+			if (unlikely(pte_alloc(mm, dst_pmd))) {
+				err = -ENOMEM;
+				break;
+			}
+
+			ret = move_pages_ptes(mm, dst_pmd, src_pmd,
+					      dst_vma, src_vma, dst_addr,
+					      src_addr, src_end - src_addr, mode);
+			if (ret < 0)
+				err = ret;
+			else
+				step_size = ret;
+		}
+
+		cond_resched();
+
+		if (fatal_signal_pending(current)) {
+			/* Do not override an error */
+			if (!err || err == -EAGAIN)
+				err = -EINTR;
+			break;
+		}
+
+		if (err) {
+			if (err == -EAGAIN)
+				continue;
+			break;
+		}
+
+		/* Proceed to the next page */
+		dst_addr += step_size;
+		src_addr += step_size;
+		moved += step_size;
+	}
+
+out_unlock:
+	up_read(&ctx->map_changing_lock);
+	uffd_move_unlock(dst_vma, src_vma);
+out:
+	VM_WARN_ON_ONCE(moved < 0);
+	VM_WARN_ON_ONCE(err > 0);
+	VM_WARN_ON_ONCE(!moved && !err);
+	return moved ? moved : err;
+}
+
+static bool vma_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags,
+		       bool wp_async)
+{
+	const struct vm_uffd_ops *ops = vma_uffd_ops(vma);
+
+	if (vma->vm_flags & (VM_DROPPABLE | VM_SHADOW_STACK))
+		return false;
+
+	if (!is_vm_hugetlb_page(vma) && (vma->vm_flags & VM_SPECIAL))
+		return false;
+
+	vm_flags &= __VM_UFFD_FLAGS;
+
+	/*
+	 * If WP is the only mode enabled and context is wp async, allow any
+	 * memory type.
+	 */
+	if (wp_async && (vm_flags == VM_UFFD_WP))
+		return true;
+
+	/* For any other mode reject VMAs that don't implement vm_uffd_ops */
+	if (!ops)
+		return false;
+
+	/*
+	 * If user requested uffd-wp but not enabled pte markers for
+	 * uffd-wp, then only anonymous memory is supported
+	 */
+	if (!uffd_supports_wp_marker() && (vm_flags & VM_UFFD_WP) &&
+	    !vma_is_anonymous(vma))
+		return false;
+
+	return ops->can_userfault(vma, vm_flags);
+}
+
+static void userfaultfd_set_vm_flags(struct vm_area_struct *vma,
+				     vm_flags_t vm_flags)
+{
+	const bool uffd_wp_changed = (vma->vm_flags ^ vm_flags) & VM_UFFD_WP;
+
+	vm_flags_reset(vma, vm_flags);
+	/*
+	 * For shared mappings, we want to enable writenotify while
+	 * userfaultfd-wp is enabled (see vma_wants_writenotify()). We'll simply
+	 * recalculate vma->vm_page_prot whenever userfaultfd-wp changes.
+	 */
+	if ((vma->vm_flags & VM_SHARED) && uffd_wp_changed)
+		vma_set_page_prot(vma);
+}
+
+static void userfaultfd_set_ctx(struct vm_area_struct *vma,
+				struct userfaultfd_ctx *ctx,
+				vm_flags_t vm_flags)
+{
+	vma_start_write(vma);
+	vma->vm_userfaultfd_ctx = (struct vm_userfaultfd_ctx){ctx};
+	userfaultfd_set_vm_flags(vma,
+				 (vma->vm_flags & ~__VM_UFFD_FLAGS) | vm_flags);
+}
+
+static void userfaultfd_reset_ctx(struct vm_area_struct *vma)
+{
+	userfaultfd_set_ctx(vma, NULL, 0);
+}
+
+static struct vm_area_struct *userfaultfd_clear_vma(struct vma_iterator *vmi,
+					     struct vm_area_struct *prev,
+					     struct vm_area_struct *vma,
+					     unsigned long start,
+					     unsigned long end)
+{
+	struct vm_area_struct *ret;
+	bool give_up_on_oom = false;
+	vma_flags_t new_vma_flags = vma->flags;
+
+	vma_flags_clear_mask(&new_vma_flags, __VMA_UFFD_FLAGS);
+
+	/*
+	 * If we are modifying only and not splitting, just give up on the merge
+	 * if OOM prevents us from merging successfully.
+	 */
+	if (start == vma->vm_start && end == vma->vm_end)
+		give_up_on_oom = true;
+
+	/* Clear the uffd bit and/or restore protnone PTEs */
+	if (userfaultfd_protected(vma)) {
+		unsigned int mm_cp_flags = 0;
+		struct mmu_gather tlb;
+
+		if (userfaultfd_wp(vma))
+			mm_cp_flags |= MM_CP_UFFD_WP_RESOLVE;
+		if (userfaultfd_rwp(vma))
+			mm_cp_flags |= MM_CP_UFFD_RWP_RESOLVE;
+		if (vma_wants_manual_pte_write_upgrade(vma))
+			mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;
+
+		tlb_gather_mmu(&tlb, vma->vm_mm);
+		change_protection(&tlb, vma, start, end, mm_cp_flags);
+		tlb_finish_mmu(&tlb);
+	}
+
+	ret = vma_modify_flags_uffd(vmi, prev, vma, start, end,
+				    &new_vma_flags, NULL_VM_UFFD_CTX,
+				    give_up_on_oom);
+
+	/*
+	 * In the vma_merge() successful mprotect-like case 8:
+	 * the next vma was merged into the current one and
+	 * the current one has not been updated yet.
+	 */
+	if (!IS_ERR(ret))
+		userfaultfd_reset_ctx(ret);
+
+	return ret;
+}
+
+/* Assumes mmap write lock taken, and mm_struct pinned. */
+static int userfaultfd_register_range(struct userfaultfd_ctx *ctx,
+			       struct vm_area_struct *vma,
+			       vm_flags_t vm_flags,
+			       unsigned long start, unsigned long end,
+			       bool wp_async)
+{
+	vma_flags_t vma_flags = legacy_to_vma_flags(vm_flags);
+	VMA_ITERATOR(vmi, ctx->mm, start);
+	struct vm_area_struct *prev = vma_prev(&vmi);
+	unsigned long vma_end;
+	vma_flags_t new_vma_flags;
+
+	if (vma->vm_start < start)
+		prev = vma;
+
+	for_each_vma_range(vmi, vma, end) {
+		cond_resched();
+
+		VM_WARN_ON_ONCE(!vma_can_userfault(vma, vm_flags, wp_async));
+		VM_WARN_ON_ONCE(vma->vm_userfaultfd_ctx.ctx &&
+				vma->vm_userfaultfd_ctx.ctx != ctx);
+		VM_WARN_ON_ONCE(!vma_test(vma, VMA_MAYWRITE_BIT));
+
+		/*
+		 * Nothing to do: this vma is already registered into this
+		 * userfaultfd and with the right tracking mode too.
+		 */
+		if (vma->vm_userfaultfd_ctx.ctx == ctx &&
+		    vma_test_all_mask(vma, vma_flags))
+			goto skip;
+
+		/*
+		 * Pre-scan in userfaultfd_register() already rejected mode
+		 * switches that would drop VM_UFFD_WP or VM_UFFD_RWP, so a
+		 * stray bit here is a bug.
+		 */
+		VM_WARN_ON_ONCE(vma->vm_userfaultfd_ctx.ctx == ctx &&
+				vma->vm_flags & (VM_UFFD_WP | VM_UFFD_RWP) & ~vm_flags);
+
+		if (vma->vm_start > start)
+			start = vma->vm_start;
+		vma_end = min(end, vma->vm_end);
+
+		new_vma_flags = vma->flags;
+		vma_flags_clear_mask(&new_vma_flags, __VMA_UFFD_FLAGS);
+		vma_flags_set_mask(&new_vma_flags, vma_flags);
+
+		vma = vma_modify_flags_uffd(&vmi, prev, vma, start, vma_end,
+					    &new_vma_flags,
+					    (struct vm_userfaultfd_ctx){ctx},
+					    /* give_up_on_oom = */false);
+		if (IS_ERR(vma))
+			return PTR_ERR(vma);
+
+		/*
+		 * In the vma_merge() successful mprotect-like case 8:
+		 * the next vma was merged into the current one and
+		 * the current one has not been updated yet.
+		 */
+		userfaultfd_set_ctx(vma, ctx, vm_flags);
+
+		if (is_vm_hugetlb_page(vma) && uffd_disable_huge_pmd_share(vma))
+			hugetlb_unshare_all_pmds(vma);
+
+skip:
+		prev = vma;
+		start = vma->vm_end;
+	}
+
+	return 0;
+}
+
+static void userfaultfd_release_new(struct userfaultfd_ctx *ctx)
+{
+	struct mm_struct *mm = ctx->mm;
+	struct vm_area_struct *vma;
+	VMA_ITERATOR(vmi, mm, 0);
+
+	/* the various vma->vm_userfaultfd_ctx still points to it */
+	mmap_write_lock(mm);
+	for_each_vma(vmi, vma) {
+		if (vma->vm_userfaultfd_ctx.ctx == ctx)
+			userfaultfd_reset_ctx(vma);
+	}
+	mmap_write_unlock(mm);
+}
+
+static void userfaultfd_release_all(struct mm_struct *mm,
+			     struct userfaultfd_ctx *ctx)
+{
+	struct vm_area_struct *vma, *prev;
+	VMA_ITERATOR(vmi, mm, 0);
+
+	if (!mmget_not_zero(mm))
+		return;
+
+	/*
+	 * Flush page faults out of all CPUs. NOTE: all page faults
+	 * must be retried without returning VM_FAULT_SIGBUS if
+	 * userfaultfd_ctx_get() succeeds but vma->vma_userfault_ctx
+	 * changes while handle_userfault released the mmap_lock. So
+	 * it's critical that released is set to true (above), before
+	 * taking the mmap_lock for writing.
+	 */
+	mmap_write_lock(mm);
+	prev = NULL;
+	for_each_vma(vmi, vma) {
+		cond_resched();
+		VM_WARN_ON_ONCE(!!vma->vm_userfaultfd_ctx.ctx ^
+				!!(vma->vm_flags & __VM_UFFD_FLAGS));
+		if (vma->vm_userfaultfd_ctx.ctx != ctx) {
+			prev = vma;
+			continue;
+		}
+
+		vma = userfaultfd_clear_vma(&vmi, prev, vma,
+					    vma->vm_start, vma->vm_end);
+		prev = vma;
+	}
+	mmap_write_unlock(mm);
+	mmput(mm);
+}
+
+static int sysctl_unprivileged_userfaultfd __read_mostly;
+
+#ifdef CONFIG_SYSCTL
+static const struct ctl_table vm_userfaultfd_table[] = {
+	{
+		.procname	= "unprivileged_userfaultfd",
+		.data		= &sysctl_unprivileged_userfaultfd,
+		.maxlen		= sizeof(sysctl_unprivileged_userfaultfd),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_ONE,
+	},
+};
+#endif
+
+static struct kmem_cache *userfaultfd_ctx_cachep __ro_after_init;
+
+struct userfaultfd_fork_ctx {
+	struct userfaultfd_ctx *orig;
+	struct userfaultfd_ctx *new;
+	struct list_head list;
+};
+
+struct userfaultfd_unmap_ctx {
+	struct userfaultfd_ctx *ctx;
+	unsigned long start;
+	unsigned long end;
+	struct list_head list;
+};
+
+struct userfaultfd_wait_queue {
+	struct uffd_msg msg;
+	wait_queue_entry_t wq;
+	struct userfaultfd_ctx *ctx;
+	bool waken;
+};
+
+struct userfaultfd_wake_range {
+	unsigned long start;
+	unsigned long len;
+};
+
+/* internal indication that UFFD_API ioctl was successfully executed */
+#define UFFD_FEATURE_INITIALIZED		(1u << 31)
+
+/*
+ * UFFDIO_SET_MODE updates ctx->features under mmap_write_lock with
+ * WRITE_ONCE; readers that run outside mmap_read_lock or the per-VMA
+ * lock (poll/read_iter/ioctl, fdinfo) must pair with READ_ONCE.
+ */
+static unsigned int userfaultfd_features(struct userfaultfd_ctx *ctx)
+{
+	return READ_ONCE(ctx->features);
+}
+
+static bool userfaultfd_is_initialized(struct userfaultfd_ctx *ctx)
+{
+	return userfaultfd_features(ctx) & UFFD_FEATURE_INITIALIZED;
+}
+
+static bool userfaultfd_wp_async_ctx(struct userfaultfd_ctx *ctx)
+{
+	return ctx && (userfaultfd_features(ctx) & UFFD_FEATURE_WP_ASYNC);
+}
+
+static bool userfaultfd_rwp_async_ctx(struct userfaultfd_ctx *ctx)
+{
+	return ctx && (userfaultfd_features(ctx) & UFFD_FEATURE_RWP_ASYNC);
+}
+
+/*
+ * Whether WP_UNPOPULATED is enabled on the uffd context.  It is only
+ * meaningful when userfaultfd_wp()==true on the vma and when it's
+ * anonymous.
+ */
+bool userfaultfd_wp_unpopulated(struct vm_area_struct *vma)
+{
+	struct userfaultfd_ctx *ctx = vma->vm_userfaultfd_ctx.ctx;
+
+	if (!ctx)
+		return false;
+
+	return userfaultfd_features(ctx) & UFFD_FEATURE_WP_UNPOPULATED;
+}
+
+static int userfaultfd_wake_function(wait_queue_entry_t *wq, unsigned mode,
+				     int wake_flags, void *key)
+{
+	struct userfaultfd_wake_range *range = key;
+	int ret;
+	struct userfaultfd_wait_queue *uwq;
+	unsigned long start, len;
+
+	uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
+	ret = 0;
+	/* len == 0 means wake all */
+	start = range->start;
+	len = range->len;
+	if (len && (start > uwq->msg.arg.pagefault.address ||
+		    start + len <= uwq->msg.arg.pagefault.address))
+		goto out;
+	WRITE_ONCE(uwq->waken, true);
+	/*
+	 * The Program-Order guarantees provided by the scheduler
+	 * ensure uwq->waken is visible before the task is woken.
+	 */
+	ret = wake_up_state(wq->private, mode);
+	if (ret) {
+		/*
+		 * Wake only once, autoremove behavior.
+		 *
+		 * After the effect of list_del_init is visible to the other
+		 * CPUs, the waitqueue may disappear from under us, see the
+		 * !list_empty_careful() in handle_userfault().
+		 *
+		 * try_to_wake_up() has an implicit smp_mb(), and the
+		 * wq->private is read before calling the extern function
+		 * "wake_up_state" (which in turns calls try_to_wake_up).
+		 */
+		list_del_init(&wq->entry);
+	}
+out:
+	return ret;
+}
+
+/**
+ * userfaultfd_ctx_get - Acquires a reference to the internal userfaultfd
+ * context.
+ * @ctx: [in] Pointer to the userfaultfd context.
+ */
+static void userfaultfd_ctx_get(struct userfaultfd_ctx *ctx)
+{
+	refcount_inc(&ctx->refcount);
+}
+
+/**
+ * userfaultfd_ctx_put - Releases a reference to the internal userfaultfd
+ * context.
+ * @ctx: [in] Pointer to userfaultfd context.
+ *
+ * The userfaultfd context reference must have been previously acquired either
+ * with userfaultfd_ctx_get() or userfaultfd_ctx_fdget().
+ */
+static void userfaultfd_ctx_put(struct userfaultfd_ctx *ctx)
+{
+	if (refcount_dec_and_test(&ctx->refcount)) {
+		VM_WARN_ON_ONCE(spin_is_locked(&ctx->fault_pending_wqh.lock));
+		VM_WARN_ON_ONCE(waitqueue_active(&ctx->fault_pending_wqh));
+		VM_WARN_ON_ONCE(spin_is_locked(&ctx->fault_wqh.lock));
+		VM_WARN_ON_ONCE(waitqueue_active(&ctx->fault_wqh));
+		VM_WARN_ON_ONCE(spin_is_locked(&ctx->event_wqh.lock));
+		VM_WARN_ON_ONCE(waitqueue_active(&ctx->event_wqh));
+		VM_WARN_ON_ONCE(spin_is_locked(&ctx->fd_wqh.lock));
+		VM_WARN_ON_ONCE(waitqueue_active(&ctx->fd_wqh));
+		mmdrop(ctx->mm);
+		kmem_cache_free(userfaultfd_ctx_cachep, ctx);
+	}
+}
+
+static inline void msg_init(struct uffd_msg *msg)
+{
+	BUILD_BUG_ON(sizeof(struct uffd_msg) != 32);
+	/*
+	 * Must use memset to zero out the paddings or kernel data is
+	 * leaked to userland.
+	 */
+	memset(msg, 0, sizeof(struct uffd_msg));
+}
+
+static inline struct uffd_msg userfault_msg(unsigned long address,
+					    unsigned long real_address,
+					    unsigned int flags,
+					    unsigned long reason,
+					    unsigned int features)
+{
+	struct uffd_msg msg;
+
+	msg_init(&msg);
+	msg.event = UFFD_EVENT_PAGEFAULT;
+
+	msg.arg.pagefault.address = (features & UFFD_FEATURE_EXACT_ADDRESS) ?
+				    real_address : address;
+
+	/*
+	 * These flags indicate why the userfault occurred:
+	 * - UFFD_PAGEFAULT_FLAG_WP indicates a write protect fault.
+	 * - UFFD_PAGEFAULT_FLAG_MINOR indicates a minor fault.
+	 * - Neither of these flags being set indicates a MISSING fault.
+	 *
+	 * Separately, UFFD_PAGEFAULT_FLAG_WRITE indicates it was a write
+	 * fault. Otherwise, it was a read fault.
+	 */
+	if (flags & FAULT_FLAG_WRITE)
+		msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WRITE;
+	if (reason & VM_UFFD_WP)
+		msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WP;
+	if (reason & VM_UFFD_RWP)
+		msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_RWP;
+	if (reason & VM_UFFD_MINOR)
+		msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_MINOR;
+	if (features & UFFD_FEATURE_THREAD_ID)
+		msg.arg.pagefault.feat.ptid = task_pid_vnr(current);
+	return msg;
+}
+
+#ifdef CONFIG_HUGETLB_PAGE
+/*
+ * Same functionality as userfaultfd_must_wait below with modifications for
+ * hugepmd ranges.
+ */
+static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
+					      struct vm_fault *vmf,
+					      unsigned long reason)
+{
+	struct vm_area_struct *vma = vmf->vma;
+	pte_t *ptep, pte;
+
+	assert_fault_locked(vmf);
+
+	ptep = hugetlb_walk(vma, vmf->address, vma_mmu_pagesize(vma));
+	if (!ptep)
+		return true;
+
+	pte = huge_ptep_get(vma->vm_mm, vmf->address, ptep);
+
+	/*
+	 * Lockless access: we're in a wait_event so it's ok if it
+	 * changes under us.
+	 */
+
+	/* Entry is still missing, wait for userspace to resolve the fault. */
+	if (huge_pte_none(pte))
+		return true;
+	/* UFFD PTE markers require userspace to resolve the fault. */
+	if (pte_is_uffd_marker(pte))
+		return true;
+	/*
+	 * Concurrent migration may have replaced the present PTE with a
+	 * non-marker swap entry between fault delivery and this lockless
+	 * re-check. huge_pte_write() on a swap entry decodes random offset
+	 * bits, so gate it on pte_present(). The migration completion path
+	 * will re-deliver the fault if it still needs userspace.
+	 */
+	if (!pte_present(pte))
+		return false;
+	/*
+	 * If VMA has UFFD WP faults enabled and WP fault, wait for userspace to
+	 * resolve the fault.
+	 */
+	if (!huge_pte_write(pte) && (reason & VM_UFFD_WP))
+		return true;
+	/*
+	 * PTE is still RW-protected (protnone with uffd bit), wait for
+	 * resolution. Plain PROT_NONE without the marker is not an RWP fault.
+	 */
+	if (pte_protnone(pte) && huge_pte_uffd(pte) && (reason & VM_UFFD_RWP))
+		return true;
+
+	return false;
+}
+#else
+static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
+					      struct vm_fault *vmf,
+					      unsigned long reason)
+{
+	/* Should never get here. */
+	VM_WARN_ON_ONCE(1);
+	return false;
+}
+#endif /* CONFIG_HUGETLB_PAGE */
+
+/*
+ * Verify the pagetables are still not ok after having registered into
+ * the fault_pending_wqh to avoid userland having to UFFDIO_WAKE any
+ * userfault that has already been resolved, if userfaultfd_read_iter and
+ * UFFDIO_COPY|ZEROPAGE are being run simultaneously on two different
+ * threads.
+ */
+static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
+					 struct vm_fault *vmf,
+					 unsigned long reason)
+{
+	struct mm_struct *mm = ctx->mm;
+	unsigned long address = vmf->address;
+	pgd_t *pgd;
+	p4d_t *p4d;
+	pud_t *pud;
+	pmd_t *pmd, _pmd;
+	pte_t *pte;
+	pte_t ptent;
+	bool ret;
+
+	assert_fault_locked(vmf);
+
+	pgd = pgd_offset(mm, address);
+	if (!pgd_present(*pgd))
+		return true;
+	p4d = p4d_offset(pgd, address);
+	if (!p4d_present(*p4d))
+		return true;
+	pud = pud_offset(p4d, address);
+	if (!pud_present(*pud))
+		return true;
+	pmd = pmd_offset(pud, address);
+again:
+	_pmd = pmdp_get_lockless(pmd);
+	if (pmd_none(_pmd))
+		return true;
+
+	/*
+	 * A race could arise which would result in a softleaf entry such as
+	 * migration entry unexpectedly being present in the PMD, so explicitly
+	 * check for this and bail out if so.
+	 */
+	if (!pmd_present(_pmd))
+		return false;
+
+	if (pmd_trans_huge(_pmd)) {
+		if (!pmd_write(_pmd) && (reason & VM_UFFD_WP))
+			return true;
+		if (pmd_protnone(_pmd) && pmd_uffd(_pmd) &&
+		    (reason & VM_UFFD_RWP))
+			return true;
+		return false;
+	}
+
+	pte = pte_offset_map(pmd, address);
+	if (!pte)
+		goto again;
+
+	/*
+	 * Lockless access: we're in a wait_event so it's ok if it
+	 * changes under us.
+	 */
+	ptent = ptep_get(pte);
+
+	ret = true;
+	/* Entry is still missing, wait for userspace to resolve the fault. */
+	if (pte_none(ptent))
+		goto out;
+	/* UFFD PTE markers require userspace to resolve the fault. */
+	if (pte_is_uffd_marker(ptent))
+		goto out;
+	/*
+	 * Concurrent swap-out / migration may have replaced the present PTE
+	 * with a non-marker swap entry between fault delivery and this
+	 * lockless re-check. pte_write() on a swap entry decodes random
+	 * offset bits, so gate it on pte_present(). The page-in path will
+	 * re-deliver the fault if it still needs userspace.
+	 */
+	if (!pte_present(ptent)) {
+		ret = false;
+		goto out;
+	}
+	/*
+	 * If VMA has UFFD WP faults enabled and WP fault, wait for userspace to
+	 * resolve the fault.
+	 */
+	if (!pte_write(ptent) && (reason & VM_UFFD_WP))
+		goto out;
+	/*
+	 * PTE is still RW-protected (protnone with uffd bit), wait for
+	 * userspace to resolve. Plain PROT_NONE without the marker is not
+	 * an RWP fault.
+	 */
+	if (pte_protnone(ptent) && pte_uffd(ptent) && (reason & VM_UFFD_RWP))
+		goto out;
+
+	ret = false;
+out:
+	pte_unmap(pte);
+	return ret;
+}
+
+static inline unsigned int userfaultfd_get_blocking_state(unsigned int flags)
+{
+	if (flags & FAULT_FLAG_INTERRUPTIBLE)
+		return TASK_INTERRUPTIBLE;
+
+	if (flags & FAULT_FLAG_KILLABLE)
+		return TASK_KILLABLE;
+
+	return TASK_UNINTERRUPTIBLE;
+}
+
+/*
+ * The locking rules involved in returning VM_FAULT_RETRY depending on
+ * FAULT_FLAG_ALLOW_RETRY, FAULT_FLAG_RETRY_NOWAIT and
+ * FAULT_FLAG_KILLABLE are not straightforward. The "Caution"
+ * recommendation in __lock_page_or_retry is not an understatement.
+ *
+ * If FAULT_FLAG_ALLOW_RETRY is set, the mmap_lock must be released
+ * before returning VM_FAULT_RETRY only if FAULT_FLAG_RETRY_NOWAIT is
+ * not set.
+ *
+ * If FAULT_FLAG_ALLOW_RETRY is set but FAULT_FLAG_KILLABLE is not
+ * set, VM_FAULT_RETRY can still be returned if and only if there are
+ * fatal_signal_pending()s, and the mmap_lock must be released before
+ * returning it.
+ */
+vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
+{
+	struct vm_area_struct *vma = vmf->vma;
+	struct mm_struct *mm = vma->vm_mm;
+	struct userfaultfd_ctx *ctx;
+	struct userfaultfd_wait_queue uwq;
+	vm_fault_t ret = VM_FAULT_SIGBUS;
+	bool must_wait;
+	unsigned int blocking_state;
+
+	/*
+	 * We don't do userfault handling for the final child pid update
+	 * and when coredumping (faults triggered by get_dump_page()).
+	 */
+	if (current->flags & (PF_EXITING|PF_DUMPCORE))
+		goto out;
+
+	assert_fault_locked(vmf);
+
+	ctx = vma->vm_userfaultfd_ctx.ctx;
+	if (!ctx)
+		goto out;
+
+	VM_WARN_ON_ONCE(ctx->mm != mm);
+
+	/* Any unrecognized flag is a bug. */
+	VM_WARN_ON_ONCE(reason & ~__VM_UFFD_FLAGS);
+	/* 0 or > 1 flags set is a bug; we expect exactly 1. */
+	VM_WARN_ON_ONCE(!reason || (reason & (reason - 1)));
+
+	if (ctx->features & UFFD_FEATURE_SIGBUS)
+		goto out;
+	if (!(vmf->flags & FAULT_FLAG_USER) && (ctx->flags & UFFD_USER_MODE_ONLY))
+		goto out;
+
+	/*
+	 * Check that we can return VM_FAULT_RETRY.
+	 *
+	 * NOTE: it should become possible to return VM_FAULT_RETRY
+	 * even if FAULT_FLAG_TRIED is set without leading to gup()
+	 * -EBUSY failures, if the userfaultfd is to be extended for
+	 * VM_UFFD_WP tracking and we intend to arm the userfault
+	 * without first stopping userland access to the memory. For
+	 * VM_UFFD_MISSING userfaults this is enough for now.
+	 */
+	if (unlikely(!(vmf->flags & FAULT_FLAG_ALLOW_RETRY))) {
+		/*
+		 * Validate the invariant that nowait must allow retry
+		 * to be sure not to return SIGBUS erroneously on
+		 * nowait invocations.
+		 */
+		VM_WARN_ON_ONCE(vmf->flags & FAULT_FLAG_RETRY_NOWAIT);
+#ifdef CONFIG_DEBUG_VM
+		if (printk_ratelimit()) {
+			pr_warn("FAULT_FLAG_ALLOW_RETRY missing %x\n",
+				vmf->flags);
+			dump_stack();
+		}
+#endif
+		goto out;
+	}
+
+	/*
+	 * Handle nowait, not much to do other than tell it to retry
+	 * and wait.
+	 */
+	ret = VM_FAULT_RETRY;
+	if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT)
+		goto out;
+
+	if (unlikely(READ_ONCE(ctx->released))) {
+		/*
+		 * If a concurrent release is detected, do not return
+		 * VM_FAULT_SIGBUS or VM_FAULT_NOPAGE, but instead always
+		 * return VM_FAULT_RETRY with lock released proactively.
+		 *
+		 * If we were to return VM_FAULT_SIGBUS here, the non
+		 * cooperative manager would be instead forced to
+		 * always call UFFDIO_UNREGISTER before it can safely
+		 * close the uffd, to avoid involuntary SIGBUS triggered.
+		 *
+		 * If we were to return VM_FAULT_NOPAGE, it would work for
+		 * the fault path, in which the lock will be released
+		 * later.  However for GUP, faultin_page() does nothing
+		 * special on NOPAGE, so GUP would spin retrying without
+		 * releasing the mmap read lock, causing possible livelock.
+		 *
+		 * Here only VM_FAULT_RETRY would make sure the mmap lock
+		 * be released immediately, so that the thread concurrently
+		 * releasing the userfault would always make progress.
+		 */
+		release_fault_lock(vmf);
+		goto out;
+	}
+
+	/* take the reference before dropping the mmap_lock */
+	userfaultfd_ctx_get(ctx);
+
+	init_waitqueue_func_entry(&uwq.wq, userfaultfd_wake_function);
+	uwq.wq.private = current;
+	uwq.msg = userfault_msg(vmf->address, vmf->real_address, vmf->flags,
+				reason, ctx->features);
+	uwq.ctx = ctx;
+	uwq.waken = false;
+
+	blocking_state = userfaultfd_get_blocking_state(vmf->flags);
+
+	/*
+	 * Take the vma lock now, in order to safely call
+	 * userfaultfd_huge_must_wait() later. Since acquiring the
+	 * (sleepable) vma lock can modify the current task state, that
+	 * must be before explicitly calling set_current_state().
+	 */
+	if (is_vm_hugetlb_page(vma))
+		hugetlb_vma_lock_read(vma);
+
+	spin_lock_irq(&ctx->fault_pending_wqh.lock);
+	/*
+	 * After the __add_wait_queue the uwq is visible to userland
+	 * through poll/read().
+	 */
+	__add_wait_queue(&ctx->fault_pending_wqh, &uwq.wq);
+	/*
+	 * The smp_mb() after __set_current_state prevents the reads
+	 * following the spin_unlock to happen before the list_add in
+	 * __add_wait_queue.
+	 */
+	set_current_state(blocking_state);
+	spin_unlock_irq(&ctx->fault_pending_wqh.lock);
+
+	if (is_vm_hugetlb_page(vma)) {
+		must_wait = userfaultfd_huge_must_wait(ctx, vmf, reason);
+		hugetlb_vma_unlock_read(vma);
+	} else {
+		must_wait = userfaultfd_must_wait(ctx, vmf, reason);
+	}
+
+	release_fault_lock(vmf);
+
+	if (likely(must_wait && !READ_ONCE(ctx->released))) {
+		wake_up_poll(&ctx->fd_wqh, EPOLLIN);
+		schedule();
+	}
+
+	__set_current_state(TASK_RUNNING);
+
+	/*
+	 * Here we race with the list_del; list_add in
+	 * userfaultfd_ctx_read(), however because we don't ever run
+	 * list_del_init() to refile across the two lists, the prev
+	 * and next pointers will never point to self. list_add also
+	 * would never let any of the two pointers to point to
+	 * self. So list_empty_careful won't risk to see both pointers
+	 * pointing to self at any time during the list refile. The
+	 * only case where list_del_init() is called is the full
+	 * removal in the wake function and there we don't re-list_add
+	 * and it's fine not to block on the spinlock. The uwq on this
+	 * kernel stack can be released after the list_del_init.
+	 */
+	if (!list_empty_careful(&uwq.wq.entry)) {
+		spin_lock_irq(&ctx->fault_pending_wqh.lock);
+		/*
+		 * No need of list_del_init(), the uwq on the stack
+		 * will be freed shortly anyway.
+		 */
+		list_del(&uwq.wq.entry);
+		spin_unlock_irq(&ctx->fault_pending_wqh.lock);
+	}
+
+	/*
+	 * ctx may go away after this if the userfault pseudo fd is
+	 * already released.
+	 */
+	userfaultfd_ctx_put(ctx);
+
+out:
+	return ret;
+}
+
+static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
+					      struct userfaultfd_wait_queue *ewq)
+{
+	struct userfaultfd_ctx *release_new_ctx;
+
+	if (WARN_ON_ONCE(current->flags & PF_EXITING))
+		goto out;
+
+	ewq->ctx = ctx;
+	init_waitqueue_entry(&ewq->wq, current);
+	release_new_ctx = NULL;
+
+	spin_lock_irq(&ctx->event_wqh.lock);
+	/*
+	 * After the __add_wait_queue the uwq is visible to userland
+	 * through poll/read().
+	 */
+	__add_wait_queue(&ctx->event_wqh, &ewq->wq);
+	for (;;) {
+		set_current_state(TASK_KILLABLE);
+		if (ewq->msg.event == 0)
+			break;
+		if (READ_ONCE(ctx->released) ||
+		    fatal_signal_pending(current)) {
+			/*
+			 * &ewq->wq may be queued in fork_event, but
+			 * __remove_wait_queue ignores the head
+			 * parameter. It would be a problem if it
+			 * didn't.
+			 */
+			__remove_wait_queue(&ctx->event_wqh, &ewq->wq);
+			if (ewq->msg.event == UFFD_EVENT_FORK) {
+				struct userfaultfd_ctx *new;
+
+				new = (struct userfaultfd_ctx *)
+					(unsigned long)
+					ewq->msg.arg.reserved.reserved1;
+				release_new_ctx = new;
+			}
+			break;
+		}
+
+		spin_unlock_irq(&ctx->event_wqh.lock);
+
+		wake_up_poll(&ctx->fd_wqh, EPOLLIN);
+		schedule();
+
+		spin_lock_irq(&ctx->event_wqh.lock);
+	}
+	__set_current_state(TASK_RUNNING);
+	spin_unlock_irq(&ctx->event_wqh.lock);
+
+	if (release_new_ctx) {
+		userfaultfd_release_new(release_new_ctx);
+		userfaultfd_ctx_put(release_new_ctx);
+	}
+
+	/*
+	 * ctx may go away after this if the userfault pseudo fd is
+	 * already released.
+	 */
+out:
+	atomic_dec(&ctx->mmap_changing);
+	VM_WARN_ON_ONCE(atomic_read(&ctx->mmap_changing) < 0);
+	userfaultfd_ctx_put(ctx);
+}
+
+static void userfaultfd_event_complete(struct userfaultfd_ctx *ctx,
+				       struct userfaultfd_wait_queue *ewq)
+{
+	ewq->msg.event = 0;
+	wake_up_locked(&ctx->event_wqh);
+	__remove_wait_queue(&ctx->event_wqh, &ewq->wq);
+}
+
+int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
+{
+	struct userfaultfd_ctx *ctx = NULL, *octx;
+	struct userfaultfd_fork_ctx *fctx;
+
+	octx = vma->vm_userfaultfd_ctx.ctx;
+	if (!octx)
+		return 0;
+
+	if (!(octx->features & UFFD_FEATURE_EVENT_FORK)) {
+		userfaultfd_reset_ctx(vma);
+		return 0;
+	}
+
+	list_for_each_entry(fctx, fcs, list)
+		if (fctx->orig == octx) {
+			ctx = fctx->new;
+			break;
+		}
+
+	if (!ctx) {
+		fctx = kmalloc_obj(*fctx);
+		if (!fctx)
+			return -ENOMEM;
+
+		ctx = kmem_cache_alloc(userfaultfd_ctx_cachep, GFP_KERNEL);
+		if (!ctx) {
+			kfree(fctx);
+			return -ENOMEM;
+		}
+
+		refcount_set(&ctx->refcount, 1);
+		ctx->flags = octx->flags;
+		ctx->features = octx->features;
+		ctx->released = false;
+		init_rwsem(&ctx->map_changing_lock);
+		atomic_set(&ctx->mmap_changing, 0);
+		ctx->mm = vma->vm_mm;
+		mmgrab(ctx->mm);
+
+		userfaultfd_ctx_get(octx);
+		down_write(&octx->map_changing_lock);
+		atomic_inc(&octx->mmap_changing);
+		up_write(&octx->map_changing_lock);
+		fctx->orig = octx;
+		fctx->new = ctx;
+		list_add_tail(&fctx->list, fcs);
+	}
+
+	vma->vm_userfaultfd_ctx.ctx = ctx;
+	return 0;
+}
+
+static void dup_fctx(struct userfaultfd_fork_ctx *fctx)
+{
+	struct userfaultfd_ctx *ctx = fctx->orig;
+	struct userfaultfd_wait_queue ewq;
+
+	msg_init(&ewq.msg);
+
+	ewq.msg.event = UFFD_EVENT_FORK;
+	ewq.msg.arg.reserved.reserved1 = (unsigned long)fctx->new;
+
+	userfaultfd_event_wait_completion(ctx, &ewq);
+}
+
+void dup_userfaultfd_complete(struct list_head *fcs)
+{
+	struct userfaultfd_fork_ctx *fctx, *n;
+
+	list_for_each_entry_safe(fctx, n, fcs, list) {
+		dup_fctx(fctx);
+		list_del(&fctx->list);
+		kfree(fctx);
+	}
+}
+
+void dup_userfaultfd_fail(struct list_head *fcs)
+{
+	struct userfaultfd_fork_ctx *fctx, *n;
+
+	/*
+	 * An error has occurred on fork, we will tear memory down, but have
+	 * allocated memory for fctx's and raised reference counts for both the
+	 * original and child contexts (and on the mm for each as a result).
+	 *
+	 * These would ordinarily be taken care of by a user handling the event,
+	 * but we are no longer doing so, so manually clean up here.
+	 *
+	 * mm tear down will take care of cleaning up VMA contexts.
+	 */
+	list_for_each_entry_safe(fctx, n, fcs, list) {
+		struct userfaultfd_ctx *octx = fctx->orig;
+		struct userfaultfd_ctx *ctx = fctx->new;
+
+		atomic_dec(&octx->mmap_changing);
+		VM_WARN_ON_ONCE(atomic_read(&octx->mmap_changing) < 0);
+		userfaultfd_ctx_put(octx);
+		userfaultfd_ctx_put(ctx);
+
+		list_del(&fctx->list);
+		kfree(fctx);
+	}
+}
+
+void mremap_userfaultfd_prep(struct vm_area_struct *vma,
+			     struct vm_userfaultfd_ctx *vm_ctx)
+{
+	struct userfaultfd_ctx *ctx;
+
+	ctx = vma->vm_userfaultfd_ctx.ctx;
+
+	if (!ctx)
+		return;
+
+	if (ctx->features & UFFD_FEATURE_EVENT_REMAP) {
+		vm_ctx->ctx = ctx;
+		userfaultfd_ctx_get(ctx);
+		down_write(&ctx->map_changing_lock);
+		atomic_inc(&ctx->mmap_changing);
+		up_write(&ctx->map_changing_lock);
+	} else {
+		/* Drop uffd context if remap feature not enabled */
+		userfaultfd_reset_ctx(vma);
+	}
+}
+
+void mremap_userfaultfd_complete(struct vm_userfaultfd_ctx *vm_ctx,
+				 unsigned long from, unsigned long to,
+				 unsigned long len)
+{
+	struct userfaultfd_ctx *ctx = vm_ctx->ctx;
+	struct userfaultfd_wait_queue ewq;
+
+	if (!ctx)
+		return;
+
+	msg_init(&ewq.msg);
+
+	ewq.msg.event = UFFD_EVENT_REMAP;
+	ewq.msg.arg.remap.from = from;
+	ewq.msg.arg.remap.to = to;
+	ewq.msg.arg.remap.len = len;
+
+	userfaultfd_event_wait_completion(ctx, &ewq);
+}
+
+void mremap_userfaultfd_fail(struct vm_userfaultfd_ctx *vm_ctx)
+{
+	struct userfaultfd_ctx *ctx = vm_ctx->ctx;
+
+	if (!ctx)
+		return;
+
+	atomic_dec(&ctx->mmap_changing);
+	VM_WARN_ON_ONCE(atomic_read(&ctx->mmap_changing) < 0);
+	userfaultfd_ctx_put(ctx);
+}
+
+bool userfaultfd_remove(struct vm_area_struct *vma,
+			unsigned long start, unsigned long end)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	struct userfaultfd_ctx *ctx;
+	struct userfaultfd_wait_queue ewq;
+
+	ctx = vma->vm_userfaultfd_ctx.ctx;
+	if (!ctx || !(ctx->features & UFFD_FEATURE_EVENT_REMOVE))
+		return true;
+
+	userfaultfd_ctx_get(ctx);
+	down_write(&ctx->map_changing_lock);
+	atomic_inc(&ctx->mmap_changing);
+	up_write(&ctx->map_changing_lock);
+	mmap_read_unlock(mm);
+
+	msg_init(&ewq.msg);
+
+	ewq.msg.event = UFFD_EVENT_REMOVE;
+	ewq.msg.arg.remove.start = start;
+	ewq.msg.arg.remove.end = end;
+
+	userfaultfd_event_wait_completion(ctx, &ewq);
+
+	return false;
+}
+
+static bool has_unmap_ctx(struct userfaultfd_ctx *ctx, struct list_head *unmaps,
+			  unsigned long start, unsigned long end)
+{
+	struct userfaultfd_unmap_ctx *unmap_ctx;
+
+	list_for_each_entry(unmap_ctx, unmaps, list)
+		if (unmap_ctx->ctx == ctx && unmap_ctx->start == start &&
+		    unmap_ctx->end == end)
+			return true;
+
+	return false;
+}
+
+int userfaultfd_unmap_prep(struct vm_area_struct *vma, unsigned long start,
+			   unsigned long end, struct list_head *unmaps)
+{
+	struct userfaultfd_unmap_ctx *unmap_ctx;
+	struct userfaultfd_ctx *ctx = vma->vm_userfaultfd_ctx.ctx;
+
+	if (!ctx || !(ctx->features & UFFD_FEATURE_EVENT_UNMAP) ||
+	    has_unmap_ctx(ctx, unmaps, start, end))
+		return 0;
+
+	unmap_ctx = kzalloc_obj(*unmap_ctx);
+	if (!unmap_ctx)
+		return -ENOMEM;
+
+	userfaultfd_ctx_get(ctx);
+	down_write(&ctx->map_changing_lock);
+	atomic_inc(&ctx->mmap_changing);
+	up_write(&ctx->map_changing_lock);
+	unmap_ctx->ctx = ctx;
+	unmap_ctx->start = start;
+	unmap_ctx->end = end;
+	list_add_tail(&unmap_ctx->list, unmaps);
+
+	return 0;
+}
+
+void userfaultfd_unmap_complete(struct mm_struct *mm, struct list_head *uf)
+{
+	struct userfaultfd_unmap_ctx *ctx, *n;
+	struct userfaultfd_wait_queue ewq;
+
+	list_for_each_entry_safe(ctx, n, uf, list) {
+		msg_init(&ewq.msg);
+
+		ewq.msg.event = UFFD_EVENT_UNMAP;
+		ewq.msg.arg.remove.start = ctx->start;
+		ewq.msg.arg.remove.end = ctx->end;
+
+		userfaultfd_event_wait_completion(ctx->ctx, &ewq);
+
+		list_del(&ctx->list);
+		kfree(ctx);
+	}
+}
+
+static int userfaultfd_release(struct inode *inode, struct file *file)
+{
+	struct userfaultfd_ctx *ctx = file->private_data;
+	struct mm_struct *mm = ctx->mm;
+	/* len == 0 means wake all */
+	struct userfaultfd_wake_range range = { .len = 0, };
+
+	WRITE_ONCE(ctx->released, true);
+
+	userfaultfd_release_all(mm, ctx);
+
+	/*
+	 * After no new page faults can wait on this fault_*wqh, flush
+	 * the last page faults that may have been already waiting on
+	 * the fault_*wqh.
+	 */
+	spin_lock_irq(&ctx->fault_pending_wqh.lock);
+	__wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL, &range);
+	__wake_up(&ctx->fault_wqh, TASK_NORMAL, 1, &range);
+	spin_unlock_irq(&ctx->fault_pending_wqh.lock);
+
+	/* Flush pending events that may still wait on event_wqh */
+	wake_up_all(&ctx->event_wqh);
+
+	wake_up_poll(&ctx->fd_wqh, EPOLLHUP);
+	userfaultfd_ctx_put(ctx);
+	return 0;
+}
+
+/* fault_pending_wqh.lock must be hold by the caller */
+static inline struct userfaultfd_wait_queue *find_userfault_in(
+		wait_queue_head_t *wqh)
+{
+	wait_queue_entry_t *wq;
+	struct userfaultfd_wait_queue *uwq;
+
+	lockdep_assert_held(&wqh->lock);
+
+	uwq = NULL;
+	if (!waitqueue_active(wqh))
+		goto out;
+	/* walk in reverse to provide FIFO behavior to read userfaults */
+	wq = list_last_entry(&wqh->head, typeof(*wq), entry);
+	uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
+out:
+	return uwq;
+}
+
+static inline struct userfaultfd_wait_queue *find_userfault(
+		struct userfaultfd_ctx *ctx)
+{
+	return find_userfault_in(&ctx->fault_pending_wqh);
+}
+
+static inline struct userfaultfd_wait_queue *find_userfault_evt(
+		struct userfaultfd_ctx *ctx)
+{
+	return find_userfault_in(&ctx->event_wqh);
+}
+
+static __poll_t userfaultfd_poll(struct file *file, poll_table *wait)
+{
+	struct userfaultfd_ctx *ctx = file->private_data;
+	__poll_t ret;
+
+	poll_wait(file, &ctx->fd_wqh, wait);
+
+	if (!userfaultfd_is_initialized(ctx))
+		return EPOLLERR;
+
+	/*
+	 * poll() never guarantees that read won't block.
+	 * userfaults can be waken before they're read().
+	 */
+	if (unlikely(!(file->f_flags & O_NONBLOCK)))
+		return EPOLLERR;
+	/*
+	 * lockless access to see if there are pending faults
+	 * __pollwait last action is the add_wait_queue but
+	 * the spin_unlock would allow the waitqueue_active to
+	 * pass above the actual list_add inside
+	 * add_wait_queue critical section. So use a full
+	 * memory barrier to serialize the list_add write of
+	 * add_wait_queue() with the waitqueue_active read
+	 * below.
+	 */
+	ret = 0;
+	smp_mb();
+	if (waitqueue_active(&ctx->fault_pending_wqh))
+		ret = EPOLLIN;
+	else if (waitqueue_active(&ctx->event_wqh))
+		ret = EPOLLIN;
+
+	return ret;
+}
+
+static const struct file_operations userfaultfd_fops;
+
+static int resolve_userfault_fork(struct userfaultfd_ctx *new,
+				  struct inode *inode,
+				  struct uffd_msg *msg)
+{
+	int fd;
+
+	fd = anon_inode_create_getfd("[userfaultfd]", &userfaultfd_fops, new,
+			O_RDONLY | (new->flags & UFFD_SHARED_FCNTL_FLAGS), inode);
+	if (fd < 0)
+		return fd;
+
+	msg->arg.reserved.reserved1 = 0;
+	msg->arg.fork.ufd = fd;
+	return 0;
+}
+
+static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
+				    struct uffd_msg *msg, struct inode *inode)
+{
+	ssize_t ret;
+	DECLARE_WAITQUEUE(wait, current);
+	struct userfaultfd_wait_queue *uwq;
+	/*
+	 * Handling fork event requires sleeping operations, so
+	 * we drop the event_wqh lock, then do these ops, then
+	 * lock it back and wake up the waiter. While the lock is
+	 * dropped the ewq may go away so we keep track of it
+	 * carefully.
+	 */
+	LIST_HEAD(fork_event);
+	struct userfaultfd_ctx *fork_nctx = NULL;
+
+	/* always take the fd_wqh lock before the fault_pending_wqh lock */
+	spin_lock_irq(&ctx->fd_wqh.lock);
+	__add_wait_queue(&ctx->fd_wqh, &wait);
+	for (;;) {
+		set_current_state(TASK_INTERRUPTIBLE);
+		spin_lock(&ctx->fault_pending_wqh.lock);
+		uwq = find_userfault(ctx);
+		if (uwq) {
+			/*
+			 * Use a seqcount to repeat the lockless check
+			 * in wake_userfault() to avoid missing
+			 * wakeups because during the refile both
+			 * waitqueue could become empty if this is the
+			 * only userfault.
+			 */
+			write_seqcount_begin(&ctx->refile_seq);
+
+			/*
+			 * The fault_pending_wqh.lock prevents the uwq
+			 * to disappear from under us.
+			 *
+			 * Refile this userfault from
+			 * fault_pending_wqh to fault_wqh, it's not
+			 * pending anymore after we read it.
+			 *
+			 * Use list_del() by hand (as
+			 * userfaultfd_wake_function also uses
+			 * list_del_init() by hand) to be sure nobody
+			 * changes __remove_wait_queue() to use
+			 * list_del_init() in turn breaking the
+			 * !list_empty_careful() check in
+			 * handle_userfault(). The uwq->wq.head list
+			 * must never be empty at any time during the
+			 * refile, or the waitqueue could disappear
+			 * from under us. The "wait_queue_head_t"
+			 * parameter of __remove_wait_queue() is unused
+			 * anyway.
+			 */
+			list_del(&uwq->wq.entry);
+			add_wait_queue(&ctx->fault_wqh, &uwq->wq);
+
+			write_seqcount_end(&ctx->refile_seq);
+
+			/* careful to always initialize msg if ret == 0 */
+			*msg = uwq->msg;
+			spin_unlock(&ctx->fault_pending_wqh.lock);
+			ret = 0;
+			break;
+		}
+		spin_unlock(&ctx->fault_pending_wqh.lock);
+
+		spin_lock(&ctx->event_wqh.lock);
+		uwq = find_userfault_evt(ctx);
+		if (uwq) {
+			*msg = uwq->msg;
+
+			if (uwq->msg.event == UFFD_EVENT_FORK) {
+				fork_nctx = (struct userfaultfd_ctx *)
+					(unsigned long)
+					uwq->msg.arg.reserved.reserved1;
+				list_move(&uwq->wq.entry, &fork_event);
+				/*
+				 * fork_nctx can be freed as soon as
+				 * we drop the lock, unless we take a
+				 * reference on it.
+				 */
+				userfaultfd_ctx_get(fork_nctx);
+				spin_unlock(&ctx->event_wqh.lock);
+				ret = 0;
+				break;
+			}
+
+			userfaultfd_event_complete(ctx, uwq);
+			spin_unlock(&ctx->event_wqh.lock);
+			ret = 0;
+			break;
+		}
+		spin_unlock(&ctx->event_wqh.lock);
+
+		if (signal_pending(current)) {
+			ret = -ERESTARTSYS;
+			break;
+		}
+		if (no_wait) {
+			ret = -EAGAIN;
+			break;
+		}
+		spin_unlock_irq(&ctx->fd_wqh.lock);
+		schedule();
+		spin_lock_irq(&ctx->fd_wqh.lock);
+	}
+	__remove_wait_queue(&ctx->fd_wqh, &wait);
+	__set_current_state(TASK_RUNNING);
+	spin_unlock_irq(&ctx->fd_wqh.lock);
+
+	if (!ret && msg->event == UFFD_EVENT_FORK) {
+		ret = resolve_userfault_fork(fork_nctx, inode, msg);
+		spin_lock_irq(&ctx->event_wqh.lock);
+		if (!list_empty(&fork_event)) {
+			/*
+			 * The fork thread didn't abort, so we can
+			 * drop the temporary refcount.
+			 */
+			userfaultfd_ctx_put(fork_nctx);
+
+			uwq = list_first_entry(&fork_event,
+					       typeof(*uwq),
+					       wq.entry);
+			/*
+			 * If fork_event list wasn't empty and in turn
+			 * the event wasn't already released by fork
+			 * (the event is allocated on fork kernel
+			 * stack), put the event back to its place in
+			 * the event_wq. fork_event head will be freed
+			 * as soon as we return so the event cannot
+			 * stay queued there no matter the current
+			 * "ret" value.
+			 */
+			list_del(&uwq->wq.entry);
+			__add_wait_queue(&ctx->event_wqh, &uwq->wq);
+
+			/*
+			 * Leave the event in the waitqueue and report
+			 * error to userland if we failed to resolve
+			 * the userfault fork.
+			 */
+			if (likely(!ret))
+				userfaultfd_event_complete(ctx, uwq);
+		} else {
+			/*
+			 * Here the fork thread aborted and the
+			 * refcount from the fork thread on fork_nctx
+			 * has already been released. We still hold
+			 * the reference we took before releasing the
+			 * lock above. If resolve_userfault_fork
+			 * failed we've to drop it because the
+			 * fork_nctx has to be freed in such case. If
+			 * it succeeded we'll hold it because the new
+			 * uffd references it.
+			 */
+			if (ret)
+				userfaultfd_ctx_put(fork_nctx);
+		}
+		spin_unlock_irq(&ctx->event_wqh.lock);
+	}
+
+	return ret;
+}
+
+static ssize_t userfaultfd_read_iter(struct kiocb *iocb, struct iov_iter *to)
+{
+	struct file *file = iocb->ki_filp;
+	struct userfaultfd_ctx *ctx = file->private_data;
+	ssize_t _ret, ret = 0;
+	struct uffd_msg msg;
+	struct inode *inode = file_inode(file);
+	bool no_wait;
+
+	if (!userfaultfd_is_initialized(ctx))
+		return -EINVAL;
+
+	no_wait = file->f_flags & O_NONBLOCK || iocb->ki_flags & IOCB_NOWAIT;
+	for (;;) {
+		if (iov_iter_count(to) < sizeof(msg))
+			return ret ? ret : -EINVAL;
+		_ret = userfaultfd_ctx_read(ctx, no_wait, &msg, inode);
+		if (_ret < 0)
+			return ret ? ret : _ret;
+		_ret = !copy_to_iter_full(&msg, sizeof(msg), to);
+		if (_ret)
+			return ret ? ret : -EFAULT;
+		ret += sizeof(msg);
+		/*
+		 * Allow to read more than one fault at time but only
+		 * block if waiting for the very first one.
+		 */
+		no_wait = true;
+	}
+}
+
+static void __wake_userfault(struct userfaultfd_ctx *ctx,
+			     struct userfaultfd_wake_range *range)
+{
+	spin_lock_irq(&ctx->fault_pending_wqh.lock);
+	/* wake all in the range and autoremove */
+	if (waitqueue_active(&ctx->fault_pending_wqh))
+		__wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL,
+				     range);
+	if (waitqueue_active(&ctx->fault_wqh))
+		__wake_up(&ctx->fault_wqh, TASK_NORMAL, 1, range);
+	spin_unlock_irq(&ctx->fault_pending_wqh.lock);
+}
+
+static __always_inline void wake_userfault(struct userfaultfd_ctx *ctx,
+					   struct userfaultfd_wake_range *range)
+{
+	unsigned seq;
+	bool need_wakeup;
+
+	/*
+	 * To be sure waitqueue_active() is not reordered by the CPU
+	 * before the pagetable update, use an explicit SMP memory
+	 * barrier here. PT lock release or mmap_read_unlock(mm) still
+	 * have release semantics that can allow the
+	 * waitqueue_active() to be reordered before the pte update.
+	 */
+	smp_mb();
+
+	/*
+	 * Use waitqueue_active because it's very frequent to
+	 * change the address space atomically even if there are no
+	 * userfaults yet. So we take the spinlock only when we're
+	 * sure we've userfaults to wake.
+	 */
+	do {
+		seq = read_seqcount_begin(&ctx->refile_seq);
+		need_wakeup = waitqueue_active(&ctx->fault_pending_wqh) ||
+			waitqueue_active(&ctx->fault_wqh);
+		cond_resched();
+	} while (read_seqcount_retry(&ctx->refile_seq, seq));
+	if (need_wakeup)
+		__wake_userfault(ctx, range);
+}
+
+static __always_inline int validate_unaligned_range(
+	struct mm_struct *mm, __u64 start, __u64 len)
+{
+	__u64 task_size = mm->task_size;
+
+	if (len & ~PAGE_MASK)
+		return -EINVAL;
+	if (!len)
+		return -EINVAL;
+	if (start >= task_size)
+		return -EINVAL;
+	if (len > task_size - start)
+		return -EINVAL;
+	if (start + len <= start)
+		return -EINVAL;
+	return 0;
+}
+
+static __always_inline int validate_range(struct mm_struct *mm,
+					  __u64 start, __u64 len)
+{
+	if (start & ~PAGE_MASK)
+		return -EINVAL;
+
+	return validate_unaligned_range(mm, start, len);
+}
+
+static int userfaultfd_register(struct userfaultfd_ctx *ctx,
+				unsigned long arg)
+{
+	struct mm_struct *mm = ctx->mm;
+	struct vm_area_struct *vma, *cur;
+	int ret;
+	struct uffdio_register uffdio_register;
+	struct uffdio_register __user *user_uffdio_register;
+	vm_flags_t vm_flags;
+	bool found;
+	bool basic_ioctls;
+	unsigned long start, end;
+	struct vma_iterator vmi;
+	bool wp_async = userfaultfd_wp_async_ctx(ctx);
+
+	user_uffdio_register = (struct uffdio_register __user *) arg;
+
+	ret = -EFAULT;
+	if (copy_from_user(&uffdio_register, user_uffdio_register,
+			   sizeof(uffdio_register)-sizeof(__u64)))
+		goto out;
+
+	ret = -EINVAL;
+	if (!uffdio_register.mode)
+		goto out;
+	if (uffdio_register.mode & ~UFFD_API_REGISTER_MODES)
+		goto out;
+	vm_flags = 0;
+	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_MISSING)
+		vm_flags |= VM_UFFD_MISSING;
+	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_WP) {
+		if (!pgtable_supports_uffd())
+			goto out;
+
+		vm_flags |= VM_UFFD_WP;
+	}
+	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_RWP) {
+		if (!pgtable_supports_uffd() || VM_UFFD_RWP == VM_NONE)
+			goto out;
+		if (!(userfaultfd_features(ctx) & UFFD_FEATURE_RWP))
+			goto out;
+		vm_flags |= VM_UFFD_RWP;
+	}
+
+	/*
+	 * WP and RWP share the uffd PTE bit and
+	 * cannot coexist in the same VMA — the bit would carry ambiguous
+	 * semantics. Reject the combination up front.
+	 */
+	if ((vm_flags & VM_UFFD_WP) && (vm_flags & VM_UFFD_RWP))
+		goto out;
+
+	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_MINOR) {
+#ifndef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
+		goto out;
+#endif
+		vm_flags |= VM_UFFD_MINOR;
+	}
+
+	ret = validate_range(mm, uffdio_register.range.start,
+			     uffdio_register.range.len);
+	if (ret)
+		goto out;
+
+	start = uffdio_register.range.start;
+	end = start + uffdio_register.range.len;
+
+	ret = -ENOMEM;
+	if (!mmget_not_zero(mm))
+		goto out;
+
+	ret = -EINVAL;
+	mmap_write_lock(mm);
+	vma_iter_init(&vmi, mm, start);
+	vma = vma_find(&vmi, end);
+	if (!vma)
+		goto out_unlock;
+
+	/*
+	 * If the first vma contains huge pages, make sure start address
+	 * is aligned to huge page size.
+	 */
+	if (is_vm_hugetlb_page(vma)) {
+		unsigned long vma_hpagesize = vma_kernel_pagesize(vma);
+
+		if (start & (vma_hpagesize - 1))
+			goto out_unlock;
+	}
+
+	/*
+	 * Search for not compatible vmas.
+	 */
+	found = false;
+	basic_ioctls = false;
+	cur = vma;
+	do {
+		cond_resched();
+
+		VM_WARN_ON_ONCE(!!cur->vm_userfaultfd_ctx.ctx ^
+				!!(cur->vm_flags & __VM_UFFD_FLAGS));
+
+		/* check not compatible vmas */
+		ret = -EINVAL;
+		if (!vma_can_userfault(cur, vm_flags, wp_async))
+			goto out_unlock;
+
+		/*
+		 * RWP uses protnone as an access-tracking marker. PROT_NONE
+		 * VMAs have vm_page_prot == PAGE_NONE, so RWP resolution
+		 * cannot make a page accessible again. Reject at register
+		 * time only: a VMA that later becomes inaccessible via
+		 * mprotect() must still be unregisterable, so this is not
+		 * part of vma_can_userfault().
+		 */
+		if ((vm_flags & VM_UFFD_RWP) && !vma_is_accessible(cur))
+			goto out_unlock;
+
+		/*
+		 * UFFDIO_COPY will fill file holes even without
+		 * PROT_WRITE. This check enforces that if this is a
+		 * MAP_SHARED, the process has write permission to the backing
+		 * file. If VM_MAYWRITE is set it also enforces that on a
+		 * MAP_SHARED vma: there is no F_WRITE_SEAL and no further
+		 * F_WRITE_SEAL can be taken until the vma is destroyed.
+		 */
+		ret = -EPERM;
+		if (unlikely(!(cur->vm_flags & VM_MAYWRITE)))
+			goto out_unlock;
+
+		/*
+		 * If this vma contains ending address, and huge pages
+		 * check alignment.
+		 */
+		if (is_vm_hugetlb_page(cur) && end <= cur->vm_end &&
+		    end > cur->vm_start) {
+			unsigned long vma_hpagesize = vma_kernel_pagesize(cur);
+
+			ret = -EINVAL;
+
+			if (end & (vma_hpagesize - 1))
+				goto out_unlock;
+		}
+		if ((vm_flags & VM_UFFD_WP) && !(cur->vm_flags & VM_MAYWRITE))
+			goto out_unlock;
+
+		/*
+		 * Check that this vma isn't already owned by a
+		 * different userfaultfd. We can't allow more than one
+		 * userfaultfd to own a single vma simultaneously or we
+		 * wouldn't know which one to deliver the userfaults to.
+		 */
+		ret = -EBUSY;
+		if (cur->vm_userfaultfd_ctx.ctx &&
+		    cur->vm_userfaultfd_ctx.ctx != ctx)
+			goto out_unlock;
+
+		/*
+		 * Mode switches that drop VM_UFFD_WP or VM_UFFD_RWP would
+		 * leave PTE markers without the flag that describes them;
+		 * subsequent mprotect() would then promote stale markers
+		 * into the other mode. Require an unregister first.
+		 */
+		if (cur->vm_userfaultfd_ctx.ctx == ctx &&
+		    cur->vm_flags & (VM_UFFD_WP | VM_UFFD_RWP) & ~vm_flags)
+			goto out_unlock;
+
+		/*
+		 * Note vmas containing huge pages
+		 */
+		if (is_vm_hugetlb_page(cur))
+			basic_ioctls = true;
+
+		found = true;
+	} for_each_vma_range(vmi, cur, end);
+	VM_WARN_ON_ONCE(!found);
+
+	ret = userfaultfd_register_range(ctx, vma, vm_flags, start, end,
+					 wp_async);
+
+out_unlock:
+	mmap_write_unlock(mm);
+	mmput(mm);
+	if (!ret) {
+		__u64 ioctls_out;
+
+		ioctls_out = basic_ioctls ? UFFD_API_RANGE_IOCTLS_BASIC :
+			UFFD_API_RANGE_IOCTLS;
+
+		/*
+		 * Declare the WP ioctl only if the WP mode is
+		 * specified and all checks passed with the range
+		 */
+		if (!(uffdio_register.mode & UFFDIO_REGISTER_MODE_WP))
+			ioctls_out &= ~((__u64)1 << _UFFDIO_WRITEPROTECT);
+
+		/* CONTINUE ioctl is only supported for MINOR ranges. */
+		if (!(uffdio_register.mode & UFFDIO_REGISTER_MODE_MINOR))
+			ioctls_out &= ~((__u64)1 << _UFFDIO_CONTINUE);
+
+		/* RWPROTECT is only supported for RWP ranges */
+		if (!(uffdio_register.mode & UFFDIO_REGISTER_MODE_RWP))
+			ioctls_out &= ~((__u64)1 << _UFFDIO_RWPROTECT);
+
+		/*
+		 * Now that we scanned all vmas we can already tell
+		 * userland which ioctls methods are guaranteed to
+		 * succeed on this range.
+		 */
+		if (put_user(ioctls_out, &user_uffdio_register->ioctls))
+			ret = -EFAULT;
+	}
+out:
+	return ret;
+}
+
+static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
+				  unsigned long arg)
+{
+	struct mm_struct *mm = ctx->mm;
+	struct vm_area_struct *vma, *prev, *cur;
+	int ret;
+	struct uffdio_range uffdio_unregister;
+	bool found;
+	unsigned long start, end, vma_end;
+	const void __user *buf = (void __user *)arg;
+	struct vma_iterator vmi;
+	bool wp_async = userfaultfd_wp_async_ctx(ctx);
+
+	ret = -EFAULT;
+	if (copy_from_user(&uffdio_unregister, buf, sizeof(uffdio_unregister)))
+		goto out;
+
+	ret = validate_range(mm, uffdio_unregister.start,
+			     uffdio_unregister.len);
+	if (ret)
+		goto out;
+
+	start = uffdio_unregister.start;
+	end = start + uffdio_unregister.len;
+
+	ret = -ENOMEM;
+	if (!mmget_not_zero(mm))
+		goto out;
+
+	mmap_write_lock(mm);
+	ret = -EINVAL;
+	vma_iter_init(&vmi, mm, start);
+	vma = vma_find(&vmi, end);
+	if (!vma)
+		goto out_unlock;
+
+	/*
+	 * If the first vma contains huge pages, make sure start address
+	 * is aligned to huge page size.
+	 */
+	if (is_vm_hugetlb_page(vma)) {
+		unsigned long vma_hpagesize = vma_kernel_pagesize(vma);
+
+		if (start & (vma_hpagesize - 1))
+			goto out_unlock;
+	}
+
+	/*
+	 * Search for not compatible vmas.
+	 */
+	found = false;
+	cur = vma;
+	do {
+		cond_resched();
+
+		VM_WARN_ON_ONCE(!!cur->vm_userfaultfd_ctx.ctx ^
+				!!(cur->vm_flags & __VM_UFFD_FLAGS));
+
+		/*
+		 * Prevent unregistering through a different userfaultfd than
+		 * the one used for registration.
+		 */
+		if (cur->vm_userfaultfd_ctx.ctx &&
+		    cur->vm_userfaultfd_ctx.ctx != ctx)
+			goto out_unlock;
+
+		/*
+		 * Check not compatible vmas, not strictly required
+		 * here as not compatible vmas cannot have an
+		 * userfaultfd_ctx registered on them, but this
+		 * provides for more strict behavior to notice
+		 * unregistration errors.
+		 */
+		if (!vma_can_userfault(cur, cur->vm_flags, wp_async))
+			goto out_unlock;
+
+		found = true;
+	} for_each_vma_range(vmi, cur, end);
+	VM_WARN_ON_ONCE(!found);
+
+	vma_iter_set(&vmi, start);
+	prev = vma_prev(&vmi);
+	if (vma->vm_start < start)
+		prev = vma;
+
+	ret = 0;
+	for_each_vma_range(vmi, vma, end) {
+		cond_resched();
+
+		/* VMA not registered with userfaultfd. */
+		if (!vma->vm_userfaultfd_ctx.ctx)
+			goto skip;
+
+		VM_WARN_ON_ONCE(vma->vm_userfaultfd_ctx.ctx != ctx);
+		VM_WARN_ON_ONCE(!vma_can_userfault(vma, vma->vm_flags, wp_async));
+		VM_WARN_ON_ONCE(!(vma->vm_flags & VM_MAYWRITE));
+
+		if (vma->vm_start > start)
+			start = vma->vm_start;
+		vma_end = min(end, vma->vm_end);
+
+		if (userfaultfd_missing(vma)) {
+			/*
+			 * Wake any concurrent pending userfault while
+			 * we unregister, so they will not hang
+			 * permanently and it avoids userland to call
+			 * UFFDIO_WAKE explicitly.
+			 */
+			struct userfaultfd_wake_range range;
+			range.start = start;
+			range.len = vma_end - start;
+			wake_userfault(vma->vm_userfaultfd_ctx.ctx, &range);
+		}
+
+		vma = userfaultfd_clear_vma(&vmi, prev, vma,
+					    start, vma_end);
+		if (IS_ERR(vma)) {
+			ret = PTR_ERR(vma);
+			break;
+		}
+
+skip:
+		prev = vma;
+		start = vma->vm_end;
+	}
+
+out_unlock:
+	mmap_write_unlock(mm);
+	mmput(mm);
+out:
+	return ret;
+}
+
+/*
+ * userfaultfd_wake may be used in combination with the
+ * UFFDIO_*_MODE_DONTWAKE to wakeup userfaults in batches.
+ */
+static int userfaultfd_wake(struct userfaultfd_ctx *ctx,
+			    unsigned long arg)
+{
+	int ret;
+	struct uffdio_range uffdio_wake;
+	struct userfaultfd_wake_range range;
+	const void __user *buf = (void __user *)arg;
+
+	ret = -EFAULT;
+	if (copy_from_user(&uffdio_wake, buf, sizeof(uffdio_wake)))
+		goto out;
+
+	ret = validate_range(ctx->mm, uffdio_wake.start, uffdio_wake.len);
+	if (ret)
+		goto out;
+
+	range.start = uffdio_wake.start;
+	range.len = uffdio_wake.len;
+
+	/*
+	 * len == 0 means wake all and we don't want to wake all here,
+	 * so check it again to be sure.
+	 */
+	VM_WARN_ON_ONCE(!range.len);
+
+	wake_userfault(ctx, &range);
+	ret = 0;
+
+out:
+	return ret;
+}
+
+static int userfaultfd_copy(struct userfaultfd_ctx *ctx,
+			    unsigned long arg)
+{
+	__s64 ret;
+	struct uffdio_copy uffdio_copy;
+	struct uffdio_copy __user *user_uffdio_copy;
+	struct userfaultfd_wake_range range;
+	uffd_flags_t flags = 0;
+
+	user_uffdio_copy = (struct uffdio_copy __user *) arg;
+
+	ret = -EAGAIN;
+	if (unlikely(atomic_read(&ctx->mmap_changing))) {
+		if (unlikely(put_user(ret, &user_uffdio_copy->copy)))
+			return -EFAULT;
+		goto out;
+	}
+
+	ret = -EFAULT;
+	if (copy_from_user(&uffdio_copy, user_uffdio_copy,
+			   /* don't copy "copy" last field */
+			   sizeof(uffdio_copy)-sizeof(__s64)))
+		goto out;
+
+	ret = validate_unaligned_range(ctx->mm, uffdio_copy.src,
+				       uffdio_copy.len);
+	if (ret)
+		goto out;
+	ret = validate_range(ctx->mm, uffdio_copy.dst, uffdio_copy.len);
+	if (ret)
+		goto out;
+
+	ret = -EINVAL;
+	if (uffdio_copy.mode & ~(UFFDIO_COPY_MODE_DONTWAKE|UFFDIO_COPY_MODE_WP))
+		goto out;
+	if (uffdio_copy.mode & UFFDIO_COPY_MODE_WP)
+		flags |= MFILL_ATOMIC_WP;
+	if (mmget_not_zero(ctx->mm)) {
+		ret = mfill_atomic_copy(ctx, uffdio_copy.dst, uffdio_copy.src,
+					uffdio_copy.len, flags);
+		mmput(ctx->mm);
+	} else {
+		return -ESRCH;
+	}
+	if (unlikely(put_user(ret, &user_uffdio_copy->copy)))
+		return -EFAULT;
+	if (ret < 0)
+		goto out;
+	VM_WARN_ON_ONCE(!ret);
+	/* len == 0 would wake all */
+	range.len = ret;
+	if (!(uffdio_copy.mode & UFFDIO_COPY_MODE_DONTWAKE)) {
+		range.start = uffdio_copy.dst;
+		wake_userfault(ctx, &range);
+	}
+	ret = range.len == uffdio_copy.len ? 0 : -EAGAIN;
+out:
+	return ret;
+}
+
+static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
+				unsigned long arg)
+{
+	__s64 ret;
+	struct uffdio_zeropage uffdio_zeropage;
+	struct uffdio_zeropage __user *user_uffdio_zeropage;
+	struct userfaultfd_wake_range range;
+
+	user_uffdio_zeropage = (struct uffdio_zeropage __user *) arg;
+
+	ret = -EAGAIN;
+	if (unlikely(atomic_read(&ctx->mmap_changing))) {
+		if (unlikely(put_user(ret, &user_uffdio_zeropage->zeropage)))
+			return -EFAULT;
+		goto out;
+	}
+
+	ret = -EFAULT;
+	if (copy_from_user(&uffdio_zeropage, user_uffdio_zeropage,
+			   /* don't copy "zeropage" last field */
+			   sizeof(uffdio_zeropage)-sizeof(__s64)))
+		goto out;
+
+	ret = validate_range(ctx->mm, uffdio_zeropage.range.start,
+			     uffdio_zeropage.range.len);
+	if (ret)
+		goto out;
+	ret = -EINVAL;
+	if (uffdio_zeropage.mode & ~UFFDIO_ZEROPAGE_MODE_DONTWAKE)
+		goto out;
+
+	if (mmget_not_zero(ctx->mm)) {
+		ret = mfill_atomic_zeropage(ctx, uffdio_zeropage.range.start,
+					    uffdio_zeropage.range.len);
+		mmput(ctx->mm);
+	} else {
+		return -ESRCH;
+	}
+	if (unlikely(put_user(ret, &user_uffdio_zeropage->zeropage)))
+		return -EFAULT;
+	if (ret < 0)
+		goto out;
+	/* len == 0 would wake all */
+	VM_WARN_ON_ONCE(!ret);
+	range.len = ret;
+	if (!(uffdio_zeropage.mode & UFFDIO_ZEROPAGE_MODE_DONTWAKE)) {
+		range.start = uffdio_zeropage.range.start;
+		wake_userfault(ctx, &range);
+	}
+	ret = range.len == uffdio_zeropage.range.len ? 0 : -EAGAIN;
+out:
+	return ret;
+}
+
+static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
+				    unsigned long arg)
+{
+	int ret;
+	struct uffdio_writeprotect uffdio_wp;
+	struct uffdio_writeprotect __user *user_uffdio_wp;
+	struct userfaultfd_wake_range range;
+	bool mode_wp, mode_dontwake;
+
+	if (atomic_read(&ctx->mmap_changing))
+		return -EAGAIN;
+
+	user_uffdio_wp = (struct uffdio_writeprotect __user *) arg;
+
+	if (copy_from_user(&uffdio_wp, user_uffdio_wp,
+			   sizeof(struct uffdio_writeprotect)))
+		return -EFAULT;
+
+	ret = validate_range(ctx->mm, uffdio_wp.range.start,
+			     uffdio_wp.range.len);
+	if (ret)
+		return ret;
+
+	if (uffdio_wp.mode & ~(UFFDIO_WRITEPROTECT_MODE_DONTWAKE |
+			       UFFDIO_WRITEPROTECT_MODE_WP))
+		return -EINVAL;
+
+	mode_wp = uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_WP;
+	mode_dontwake = uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_DONTWAKE;
+
+	if (mode_wp && mode_dontwake)
+		return -EINVAL;
+
+	if (mmget_not_zero(ctx->mm)) {
+		ret = mwriteprotect_range(ctx, uffdio_wp.range.start,
+					  uffdio_wp.range.len, mode_wp);
+		mmput(ctx->mm);
+	} else {
+		return -ESRCH;
+	}
+
+	if (ret)
+		return ret;
+
+	if (!mode_wp && !mode_dontwake) {
+		range.start = uffdio_wp.range.start;
+		range.len = uffdio_wp.range.len;
+		wake_userfault(ctx, &range);
+	}
+	return ret;
+}
+
+static int userfaultfd_rwprotect(struct userfaultfd_ctx *ctx,
+				 unsigned long arg)
+{
+	int ret;
+	struct uffdio_rwprotect uffdio_rwp;
+	struct userfaultfd_wake_range range;
+	bool mode_rwp, mode_dontwake;
+
+	if (atomic_read(&ctx->mmap_changing))
+		return -EAGAIN;
+
+	if (copy_from_user(&uffdio_rwp, (void __user *)arg,
+			   sizeof(uffdio_rwp)))
+		return -EFAULT;
+
+	ret = validate_range(ctx->mm, uffdio_rwp.range.start,
+			     uffdio_rwp.range.len);
+	if (ret)
+		return ret;
+
+	if (uffdio_rwp.mode & ~(UFFDIO_RWPROTECT_MODE_DONTWAKE |
+				UFFDIO_RWPROTECT_MODE_RWP))
+		return -EINVAL;
+
+	mode_rwp = uffdio_rwp.mode & UFFDIO_RWPROTECT_MODE_RWP;
+	mode_dontwake = uffdio_rwp.mode & UFFDIO_RWPROTECT_MODE_DONTWAKE;
+
+	if (mode_rwp && mode_dontwake)
+		return -EINVAL;
+
+	if (mmget_not_zero(ctx->mm)) {
+		ret = mrwprotect_range(ctx, uffdio_rwp.range.start,
+				       uffdio_rwp.range.len, mode_rwp);
+		mmput(ctx->mm);
+	} else {
+		return -ESRCH;
+	}
+
+	if (ret)
+		return ret;
+
+	if (!mode_rwp && !mode_dontwake) {
+		range.start = uffdio_rwp.range.start;
+		range.len = uffdio_rwp.range.len;
+		wake_userfault(ctx, &range);
+	}
+	return ret;
+}
+
+/* Subset of UFFD_API_FEATURES actually supported by this kernel/arch */
+static __u64 uffd_api_available_features(void)
+{
+	__u64 f = UFFD_API_FEATURES;
+
+	if (!IS_ENABLED(CONFIG_HAVE_ARCH_USERFAULTFD_MINOR))
+		f &= ~(UFFD_FEATURE_MINOR_HUGETLBFS | UFFD_FEATURE_MINOR_SHMEM);
+	if (!pgtable_supports_uffd())
+		f &= ~UFFD_FEATURE_PAGEFAULT_FLAG_WP;
+	if (!uffd_supports_wp_marker())
+		f &= ~(UFFD_FEATURE_WP_HUGETLBFS_SHMEM |
+		       UFFD_FEATURE_WP_UNPOPULATED |
+		       UFFD_FEATURE_WP_ASYNC);
+	/*
+	 * RWP needs both PROT_NONE support and the uffd PTE bit. The
+	 * VM_UFFD_RWP check covers compile-time unavailability; the
+	 * pgtable_supports_uffd() check covers runtime (e.g. riscv
+	 * without the SVRSW60T59B extension) where the PTE bit is declared
+	 * but not actually usable.
+	 */
+	if (VM_UFFD_RWP == VM_NONE || !pgtable_supports_uffd())
+		f &= ~(UFFD_FEATURE_RWP | UFFD_FEATURE_RWP_ASYNC);
+	return f;
+}
+
+/* Async features that can be toggled at runtime via UFFDIO_SET_MODE */
+#define UFFD_FEATURE_TOGGLEABLE	UFFD_FEATURE_RWP_ASYNC
+
+static int userfaultfd_set_mode(struct userfaultfd_ctx *ctx,
+				unsigned long arg)
+{
+	struct uffdio_set_mode mode;
+	struct mm_struct *mm = ctx->mm;
+
+	if (copy_from_user(&mode, (void __user *)arg, sizeof(mode)))
+		return -EFAULT;
+
+	/* enable and disable must not overlap */
+	if (mode.enable & mode.disable)
+		return -EINVAL;
+
+	/* only toggleable features that this kernel/arch actually supports */
+	if ((mode.enable | mode.disable) &
+	    ~(uffd_api_available_features() & UFFD_FEATURE_TOGGLEABLE))
+		return -EINVAL;
+
+	/* RWP_ASYNC can only be enabled on contexts that negotiated RWP */
+	if ((mode.enable & UFFD_FEATURE_RWP_ASYNC) &&
+	    !(userfaultfd_features(ctx) & UFFD_FEATURE_RWP))
+		return -EINVAL;
+
+	if (!mmget_not_zero(mm))
+		return -ESRCH;
+
+	/*
+	 * Drain in-flight faults before flipping features. mmap_write_lock()
+	 * blocks new mmap_read_lock() callers, but per-VMA locked faults
+	 * (lock_vma_under_rcu() + FAULT_FLAG_VMA_LOCK) that acquired before
+	 * this point keep running. Calling vma_start_write() on each UFFD-
+	 * armed VMA waits for those readers to drop, so no in-flight fault
+	 * can observe the old features after mmap_write_unlock().
+	 */
+	mmap_write_lock(mm);
+	{
+		struct vm_area_struct *vma;
+		VMA_ITERATOR(vmi, mm, 0);
+
+		for_each_vma(vmi, vma) {
+			if (vma->vm_userfaultfd_ctx.ctx == ctx)
+				vma_start_write(vma);
+		}
+	}
+	/*
+	 * Single WRITE_ONCE so lockless readers (fdinfo, poll/read_iter
+	 * via userfaultfd_is_initialized(), and the userfaultfd_features()
+	 * helper used elsewhere) can't observe a mid-RMW intermediate
+	 * value. Hot-path readers already serialise through the mmap lock
+	 * + vma_start_write() drain above, so their load doesn't need an
+	 * annotation.
+	 */
+	WRITE_ONCE(ctx->features,
+		   (ctx->features | mode.enable) & ~mode.disable);
+	mmap_write_unlock(mm);
+
+	/*
+	 * If switching to async, wake threads blocked in handle_userfault().
+	 * They will retry the fault and auto-resolve under the new mode.
+	 * len=0 means wake all pending faults on this context.
+	 */
+	if (mode.enable & UFFD_FEATURE_RWP_ASYNC) {
+		struct userfaultfd_wake_range range = { .len = 0 };
+
+		spin_lock_irq(&ctx->fault_pending_wqh.lock);
+		__wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL,
+				     &range);
+		__wake_up(&ctx->fault_wqh, TASK_NORMAL, 1, &range);
+		spin_unlock_irq(&ctx->fault_pending_wqh.lock);
+	}
+
+	mmput(mm);
+	return 0;
+}
+
+static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg)
+{
+	__s64 ret;
+	struct uffdio_continue uffdio_continue;
+	struct uffdio_continue __user *user_uffdio_continue;
+	struct userfaultfd_wake_range range;
+	uffd_flags_t flags = 0;
+
+	user_uffdio_continue = (struct uffdio_continue __user *)arg;
+
+	ret = -EAGAIN;
+	if (unlikely(atomic_read(&ctx->mmap_changing))) {
+		if (unlikely(put_user(ret, &user_uffdio_continue->mapped)))
+			return -EFAULT;
+		goto out;
+	}
+
+	ret = -EFAULT;
+	if (copy_from_user(&uffdio_continue, user_uffdio_continue,
+			   /* don't copy the output fields */
+			   sizeof(uffdio_continue) - (sizeof(__s64))))
+		goto out;
+
+	ret = validate_range(ctx->mm, uffdio_continue.range.start,
+			     uffdio_continue.range.len);
+	if (ret)
+		goto out;
+
+	ret = -EINVAL;
+	if (uffdio_continue.mode & ~(UFFDIO_CONTINUE_MODE_DONTWAKE |
+				     UFFDIO_CONTINUE_MODE_WP))
+		goto out;
+	if (uffdio_continue.mode & UFFDIO_CONTINUE_MODE_WP)
+		flags |= MFILL_ATOMIC_WP;
+
+	if (mmget_not_zero(ctx->mm)) {
+		ret = mfill_atomic_continue(ctx, uffdio_continue.range.start,
+					    uffdio_continue.range.len, flags);
+		mmput(ctx->mm);
+	} else {
+		return -ESRCH;
+	}
+
+	if (unlikely(put_user(ret, &user_uffdio_continue->mapped)))
+		return -EFAULT;
+	if (ret < 0)
+		goto out;
+
+	/* len == 0 would wake all */
+	VM_WARN_ON_ONCE(!ret);
+	range.len = ret;
+	if (!(uffdio_continue.mode & UFFDIO_CONTINUE_MODE_DONTWAKE)) {
+		range.start = uffdio_continue.range.start;
+		wake_userfault(ctx, &range);
+	}
+	ret = range.len == uffdio_continue.range.len ? 0 : -EAGAIN;
+
+out:
+	return ret;
+}
+
+static inline int userfaultfd_poison(struct userfaultfd_ctx *ctx, unsigned long arg)
+{
+	__s64 ret;
+	struct uffdio_poison uffdio_poison;
+	struct uffdio_poison __user *user_uffdio_poison;
+	struct userfaultfd_wake_range range;
+
+	user_uffdio_poison = (struct uffdio_poison __user *)arg;
+
+	ret = -EAGAIN;
+	if (unlikely(atomic_read(&ctx->mmap_changing))) {
+		if (unlikely(put_user(ret, &user_uffdio_poison->updated)))
+			return -EFAULT;
+		goto out;
+	}
+
+	ret = -EFAULT;
+	if (copy_from_user(&uffdio_poison, user_uffdio_poison,
+			   /* don't copy the output fields */
+			   sizeof(uffdio_poison) - (sizeof(__s64))))
+		goto out;
+
+	ret = validate_range(ctx->mm, uffdio_poison.range.start,
+			     uffdio_poison.range.len);
+	if (ret)
+		goto out;
+
+	ret = -EINVAL;
+	if (uffdio_poison.mode & ~UFFDIO_POISON_MODE_DONTWAKE)
+		goto out;
+
+	if (mmget_not_zero(ctx->mm)) {
+		ret = mfill_atomic_poison(ctx, uffdio_poison.range.start,
+					  uffdio_poison.range.len, 0);
+		mmput(ctx->mm);
+	} else {
+		return -ESRCH;
+	}
+
+	if (unlikely(put_user(ret, &user_uffdio_poison->updated)))
+		return -EFAULT;
+	if (ret < 0)
+		goto out;
+
+	/* len == 0 would wake all */
+	VM_WARN_ON_ONCE(!ret);
+	range.len = ret;
+	if (!(uffdio_poison.mode & UFFDIO_POISON_MODE_DONTWAKE)) {
+		range.start = uffdio_poison.range.start;
+		wake_userfault(ctx, &range);
+	}
+	ret = range.len == uffdio_poison.range.len ? 0 : -EAGAIN;
+
+out:
+	return ret;
+}
+
+bool userfaultfd_wp_async(struct vm_area_struct *vma)
+{
+	return userfaultfd_wp_async_ctx(vma->vm_userfaultfd_ctx.ctx);
+}
+
+bool userfaultfd_rwp_async(struct vm_area_struct *vma)
+{
+	return userfaultfd_rwp_async_ctx(vma->vm_userfaultfd_ctx.ctx);
+}
+
+static inline unsigned int uffd_ctx_features(__u64 user_features)
+{
+	/*
+	 * For the current set of features the bits just coincide. Set
+	 * UFFD_FEATURE_INITIALIZED to mark the features as enabled.
+	 */
+	return (unsigned int)user_features | UFFD_FEATURE_INITIALIZED;
+}
+
+static int userfaultfd_move(struct userfaultfd_ctx *ctx,
+			    unsigned long arg)
+{
+	__s64 ret;
+	struct uffdio_move uffdio_move;
+	struct uffdio_move __user *user_uffdio_move;
+	struct userfaultfd_wake_range range;
+	struct mm_struct *mm = ctx->mm;
+
+	user_uffdio_move = (struct uffdio_move __user *) arg;
+
+	ret = -EAGAIN;
+	if (unlikely(atomic_read(&ctx->mmap_changing))) {
+		if (unlikely(put_user(ret, &user_uffdio_move->move)))
+			return -EFAULT;
+		goto out;
+	}
+
+	if (copy_from_user(&uffdio_move, user_uffdio_move,
+			   /* don't copy "move" last field */
+			   sizeof(uffdio_move)-sizeof(__s64)))
+		return -EFAULT;
+
+	/* Do not allow cross-mm moves. */
+	if (mm != current->mm)
+		return -EINVAL;
+
+	ret = validate_range(mm, uffdio_move.dst, uffdio_move.len);
+	if (ret)
+		return ret;
+
+	ret = validate_range(mm, uffdio_move.src, uffdio_move.len);
+	if (ret)
+		return ret;
+
+	if (uffdio_move.mode & ~(UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES|
+				 UFFDIO_MOVE_MODE_DONTWAKE))
+		return -EINVAL;
+
+	if (mmget_not_zero(mm)) {
+		ret = move_pages(ctx, uffdio_move.dst, uffdio_move.src,
+				 uffdio_move.len, uffdio_move.mode);
+		mmput(mm);
+	} else {
+		return -ESRCH;
+	}
+
+	if (unlikely(put_user(ret, &user_uffdio_move->move)))
+		return -EFAULT;
+	if (ret < 0)
+		goto out;
+
+	/* len == 0 would wake all */
+	VM_WARN_ON(!ret);
+	range.len = ret;
+	if (!(uffdio_move.mode & UFFDIO_MOVE_MODE_DONTWAKE)) {
+		range.start = uffdio_move.dst;
+		wake_userfault(ctx, &range);
+	}
+	ret = range.len == uffdio_move.len ? 0 : -EAGAIN;
+
+out:
+	return ret;
+}
+
+/*
+ * userland asks for a certain API version and we return which bits
+ * and ioctl commands are implemented in this kernel for such API
+ * version or -EINVAL if unknown.
+ */
+static int userfaultfd_api(struct userfaultfd_ctx *ctx,
+			   unsigned long arg)
+{
+	struct uffdio_api uffdio_api;
+	void __user *buf = (void __user *)arg;
+	unsigned int ctx_features;
+	int ret;
+	__u64 features;
+
+	ret = -EFAULT;
+	if (copy_from_user(&uffdio_api, buf, sizeof(uffdio_api)))
+		goto out;
+	features = uffdio_api.features;
+	ret = -EINVAL;
+	if (uffdio_api.api != UFFD_API)
+		goto err_out;
+	ret = -EPERM;
+	if ((features & UFFD_FEATURE_EVENT_FORK) && !capable(CAP_SYS_PTRACE))
+		goto err_out;
+
+	/* WP_ASYNC relies on WP_UNPOPULATED, choose it unconditionally */
+	if (features & UFFD_FEATURE_WP_ASYNC)
+		features |= UFFD_FEATURE_WP_UNPOPULATED;
+
+	ret = -EINVAL;
+	/* RWP_ASYNC requires RWP */
+	if ((features & UFFD_FEATURE_RWP_ASYNC) &&
+	    !(features & UFFD_FEATURE_RWP))
+		goto err_out;
+
+	/* report all available features and ioctls to userland */
+	uffdio_api.features = uffd_api_available_features();
+
+	ret = -EINVAL;
+	if (features & ~uffdio_api.features)
+		goto err_out;
+
+	uffdio_api.ioctls = UFFD_API_IOCTLS;
+	ret = -EFAULT;
+	if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
+		goto out;
+
+	/* only enable the requested features for this uffd context */
+	ctx_features = uffd_ctx_features(features);
+	ret = -EINVAL;
+	if (cmpxchg(&ctx->features, 0, ctx_features) != 0)
+		goto err_out;
+
+	ret = 0;
+out:
+	return ret;
+err_out:
+	memset(&uffdio_api, 0, sizeof(uffdio_api));
+	if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
+		ret = -EFAULT;
+	goto out;
+}
+
+static long userfaultfd_ioctl(struct file *file, unsigned cmd,
+			      unsigned long arg)
+{
+	int ret = -EINVAL;
+	struct userfaultfd_ctx *ctx = file->private_data;
+
+	if (cmd != UFFDIO_API && !userfaultfd_is_initialized(ctx))
+		return -EINVAL;
+
+	switch (cmd) {
+	case UFFDIO_API:
+		ret = userfaultfd_api(ctx, arg);
+		break;
+	case UFFDIO_REGISTER:
+		ret = userfaultfd_register(ctx, arg);
+		break;
+	case UFFDIO_UNREGISTER:
+		ret = userfaultfd_unregister(ctx, arg);
+		break;
+	case UFFDIO_WAKE:
+		ret = userfaultfd_wake(ctx, arg);
+		break;
+	case UFFDIO_COPY:
+		ret = userfaultfd_copy(ctx, arg);
+		break;
+	case UFFDIO_ZEROPAGE:
+		ret = userfaultfd_zeropage(ctx, arg);
+		break;
+	case UFFDIO_MOVE:
+		ret = userfaultfd_move(ctx, arg);
+		break;
+	case UFFDIO_WRITEPROTECT:
+		ret = userfaultfd_writeprotect(ctx, arg);
+		break;
+	case UFFDIO_CONTINUE:
+		ret = userfaultfd_continue(ctx, arg);
+		break;
+	case UFFDIO_POISON:
+		ret = userfaultfd_poison(ctx, arg);
+		break;
+	case UFFDIO_RWPROTECT:
+		ret = userfaultfd_rwprotect(ctx, arg);
+		break;
+	case UFFDIO_SET_MODE:
+		ret = userfaultfd_set_mode(ctx, arg);
+		break;
+	}
+	return ret;
+}
+
+#ifdef CONFIG_PROC_FS
+static void userfaultfd_show_fdinfo(struct seq_file *m, struct file *f)
+{
+	struct userfaultfd_ctx *ctx = f->private_data;
+	wait_queue_entry_t *wq;
+	unsigned long pending = 0, total = 0;
+
+	spin_lock_irq(&ctx->fault_pending_wqh.lock);
+	list_for_each_entry(wq, &ctx->fault_pending_wqh.head, entry) {
+		pending++;
+		total++;
+	}
+	list_for_each_entry(wq, &ctx->fault_wqh.head, entry) {
+		total++;
+	}
+	spin_unlock_irq(&ctx->fault_pending_wqh.lock);
+
+	/*
+	 * If more protocols will be added, there will be all shown
+	 * separated by a space. Like this:
+	 *	protocols: aa:... bb:...
+	 */
+	seq_printf(m, "pending:\t%lu\ntotal:\t%lu\nAPI:\t%Lx:%x:%Lx\n",
+		   pending, total, UFFD_API, userfaultfd_features(ctx),
+		   UFFD_API_IOCTLS|UFFD_API_RANGE_IOCTLS);
+}
+#endif
+
+static const struct file_operations userfaultfd_fops = {
+#ifdef CONFIG_PROC_FS
+	.show_fdinfo	= userfaultfd_show_fdinfo,
+#endif
+	.release	= userfaultfd_release,
+	.poll		= userfaultfd_poll,
+	.read_iter	= userfaultfd_read_iter,
+	.unlocked_ioctl = userfaultfd_ioctl,
+	.compat_ioctl	= compat_ptr_ioctl,
+	.llseek		= noop_llseek,
+};
+
+static void init_once_userfaultfd_ctx(void *mem)
+{
+	struct userfaultfd_ctx *ctx = (struct userfaultfd_ctx *) mem;
+
+	init_waitqueue_head(&ctx->fault_pending_wqh);
+	init_waitqueue_head(&ctx->fault_wqh);
+	init_waitqueue_head(&ctx->event_wqh);
+	init_waitqueue_head(&ctx->fd_wqh);
+	seqcount_spinlock_init(&ctx->refile_seq, &ctx->fault_pending_wqh.lock);
+}
+
+static int new_userfaultfd(int flags)
+{
+	struct userfaultfd_ctx *ctx __free(kfree) = NULL;
+
+	VM_WARN_ON_ONCE(!current->mm);
+
+	/* Check the UFFD_* constants for consistency. */
+	BUILD_BUG_ON(UFFD_USER_MODE_ONLY & UFFD_SHARED_FCNTL_FLAGS);
+
+	if (flags & ~(UFFD_SHARED_FCNTL_FLAGS | UFFD_USER_MODE_ONLY))
+		return -EINVAL;
+
+	ctx = kmem_cache_alloc(userfaultfd_ctx_cachep, GFP_KERNEL);
+	if (!ctx)
+		return -ENOMEM;
+
+	refcount_set(&ctx->refcount, 1);
+	ctx->flags = flags;
+	ctx->features = 0;
+	ctx->released = false;
+	init_rwsem(&ctx->map_changing_lock);
+	atomic_set(&ctx->mmap_changing, 0);
+	ctx->mm = current->mm;
+
+	FD_PREPARE(fdf, flags & UFFD_SHARED_FCNTL_FLAGS,
+		   anon_inode_create_getfile("[userfaultfd]", &userfaultfd_fops, ctx,
+					     O_RDONLY | (flags & UFFD_SHARED_FCNTL_FLAGS),
+					     NULL));
+	if (fdf.err)
+		return fdf.err;
+
+	/* prevent the mm struct to be freed */
+	mmgrab(ctx->mm);
+	fd_prepare_file(fdf)->f_mode |= FMODE_NOWAIT;
+	retain_and_null_ptr(ctx);
+	return fd_publish(fdf);
+}
+
+static inline bool userfaultfd_syscall_allowed(int flags)
+{
+	/* Userspace-only page faults are always allowed */
+	if (flags & UFFD_USER_MODE_ONLY)
+		return true;
+
+	/*
+	 * The user is requesting a userfaultfd which can handle kernel faults.
+	 * Privileged users are always allowed to do this.
+	 */
+	if (capable(CAP_SYS_PTRACE))
+		return true;
+
+	/* Otherwise, access to kernel fault handling is sysctl controlled. */
+	return sysctl_unprivileged_userfaultfd;
+}
+
+SYSCALL_DEFINE1(userfaultfd, int, flags)
+{
+	if (!userfaultfd_syscall_allowed(flags))
+		return -EPERM;
+
+	return new_userfaultfd(flags);
+}
+
+static long userfaultfd_dev_ioctl(struct file *file, unsigned int cmd, unsigned long flags)
+{
+	if (cmd != USERFAULTFD_IOC_NEW)
+		return -EINVAL;
+
+	return new_userfaultfd(flags);
+}
+
+static const struct file_operations userfaultfd_dev_fops = {
+	.unlocked_ioctl = userfaultfd_dev_ioctl,
+	.compat_ioctl = userfaultfd_dev_ioctl,
+	.owner = THIS_MODULE,
+	.llseek = noop_llseek,
+};
+
+static struct miscdevice userfaultfd_misc = {
+	.minor = MISC_DYNAMIC_MINOR,
+	.name = "userfaultfd",
+	.fops = &userfaultfd_dev_fops
+};
+
+static int __init userfaultfd_init(void)
+{
+	int ret;
+
+	ret = misc_register(&userfaultfd_misc);
+	if (ret)
+		return ret;
+
+	userfaultfd_ctx_cachep = kmem_cache_create("userfaultfd_ctx_cache",
+						sizeof(struct userfaultfd_ctx),
+						0,
+						SLAB_HWCACHE_ALIGN|SLAB_PANIC,
+						init_once_userfaultfd_ctx);
+#ifdef CONFIG_SYSCTL
+	register_sysctl_init("vm", vm_userfaultfd_table);
+#endif
+	return 0;
+}
+__initcall(userfaultfd_init);