Merge tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-tracegrafted

Pull tracing fixes from Steven Rostedt:

 - Fix error output of boot instance creation failure

   Currently if a boot instance creation fails, instead of printing out
   the name of the instance that failed, it prints "(null)". That is
   because it prints "cur_str" that had already been processed by
   strsep(). Print the saved name instead.

   While at it, print the error code of the failure.

 - Fix use-after-free for same named historgrams

   Histograms can be named so that they can be used in multiple events.
   But if the named histogram has a variable attached, the second event
   that uses the named histogram which duplicates it and needs to free
   the original after duplication leaves the old variable in place and
   still visible. If another histogram uses than variable, it will use
   the stale one which will try to reference the freed duplicate
   histogram and crash the kernel.

   Free the duplicate variables along with the duplicated histogram
   data.

 - Check return value of kthread_run() in event self test

   The events self tests uses a kthread for testing but does not check
   if it succeeded in creating a kthread. If the kthread creation were
   to fail, the code will still try to call kthread_stop() on the error
   returned.

 - Fix race between reading trace_pipe and updating subbuffer size

   If a user is reading the trace_pipe file at the same time they update
   the ring buffer sub-buffer size, can cause the trace_pipe read to
   read stale data. Add trace_access_lock() around updating the ring
   buffer sub-buffer size.

 - Fix eventfs_inode on failure path in creation of the events directory

   In the creation of the "events" directory, if after allocating the
   eventfs_inode a failure is detected, it calls cleanup_ei() which
   calls free_ei(). The free_ei() will test if eventfs_inode being freed
   has no children. It is a bug if it does. But on the failure case of
   the creation of the "events" directory, the children lists have not
   yet been initialized and the free will trigger a warning because
   list_empty() on an uninitialized list returns false.

   Move the initialization into init_ei() where it makes more sense and
   makes sure that a created eventfs_inode has its lists initialized
   upon creation.

 - Check return value of kthread_run() in ftrace direct sample code

   The sample code that shows how to use the ftrace direct calls does
   not test the return of kthread_run() to see if it succeeds. Return a
   failure if the kthread_run() doesn't succeed.

 - Clear user events state on fork in case of alloc failure

   On fork, the child gets a pointer to the parent's user events state.
   It makes a copy of it then updates the child's pointer to it. But if
   the allocation fails, the duplication function leaves the child with
   a pointer to its parent's descriptor. When the child cleans up its
   data, it will free the parent's descriptor while the parent is still
   using it.

   In the duplication function, set the child's user_event_mm to NULL
   before testing if the allocation succeeded, and when it exits it will
   not free the parent's descriptor.

 - Fix retry exhaustion in simple ring buffer reader swap

   simple_ring_buffer_swap_reader_page() starts with retry set to 8 and
   post-decrements it only after a failed link replacement. On the final
   attempt, a successful replacement leaves retry at zero, while a
   failed replacement leaves it at -1.

   But the check for success expects the retry value to be non-zero and
   exits with an error on zero. This is the opposite result. Fix it.

 - Fail nicely when the remote swap_reader_page() returns an error

   Currently, if the swap_reader_page() of a remote buffer fails, it
   triggers a WARN_ON_ONCE() and continues normally. Instead, have it
   exit with an error and a pr_warn() print instead of a full WARNING.

* tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace:
  ring-buffer: Stop remote reader update when page swap fails
  tracing: Fix retry exhaustion in simple ring buffer reader swap
  tracing/user_events: Clear copied tracing state before fork duplication
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-multi-modify
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-modify
  eventfs: Initialize ei->children and ei->list in init_ei()
  tracing: Fix use-after-free in trace_pipe read on sub-buffer order change
  tracing: Fix crash passing ERR_PTR to kthread_stop()
  tracing: Fix use-after-free with same-name named triggers
  tracing: Fix logged instance name on creation failure

1 files changed, 549 insertions, 0 deletions

diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
new file mode 100644
index 000000000..0236b6b97
--- /dev/null
+++ b/fs/ext4/page-io.c
@@ -0,0 +1,549 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * linux/fs/ext4/page-io.c
+ *
+ * This contains the new page_io functions for ext4
+ *
+ * Written by Theodore Ts'o, 2010.
+ */
+
+#include <linux/blk-crypto.h>
+#include <linux/fs.h>
+#include <linux/time.h>
+#include <linux/highuid.h>
+#include <linux/pagemap.h>
+#include <linux/quotaops.h>
+#include <linux/string.h>
+#include <linux/buffer_head.h>
+#include <linux/writeback.h>
+#include <linux/mpage.h>
+#include <linux/namei.h>
+#include <linux/uio.h>
+#include <linux/bio.h>
+#include <linux/workqueue.h>
+#include <linux/kernel.h>
+#include <linux/slab.h>
+#include <linux/mm.h>
+#include <linux/sched/mm.h>
+
+#include "ext4_jbd2.h"
+#include "xattr.h"
+#include "acl.h"
+
+static struct kmem_cache *io_end_cachep;
+static struct kmem_cache *io_end_vec_cachep;
+
+int __init ext4_init_pageio(void)
+{
+	io_end_cachep = KMEM_CACHE(ext4_io_end, SLAB_RECLAIM_ACCOUNT);
+	if (io_end_cachep == NULL)
+		return -ENOMEM;
+
+	io_end_vec_cachep = KMEM_CACHE(ext4_io_end_vec, 0);
+	if (io_end_vec_cachep == NULL) {
+		kmem_cache_destroy(io_end_cachep);
+		return -ENOMEM;
+	}
+	return 0;
+}
+
+void ext4_exit_pageio(void)
+{
+	kmem_cache_destroy(io_end_cachep);
+	kmem_cache_destroy(io_end_vec_cachep);
+}
+
+struct ext4_io_end_vec *ext4_alloc_io_end_vec(ext4_io_end_t *io_end)
+{
+	struct ext4_io_end_vec *io_end_vec;
+
+	io_end_vec = kmem_cache_zalloc(io_end_vec_cachep, GFP_NOFS);
+	if (!io_end_vec)
+		return ERR_PTR(-ENOMEM);
+	INIT_LIST_HEAD(&io_end_vec->list);
+	list_add_tail(&io_end_vec->list, &io_end->list_vec);
+	return io_end_vec;
+}
+
+static void ext4_free_io_end_vec(ext4_io_end_t *io_end)
+{
+	struct ext4_io_end_vec *io_end_vec, *tmp;
+
+	if (list_empty(&io_end->list_vec))
+		return;
+	list_for_each_entry_safe(io_end_vec, tmp, &io_end->list_vec, list) {
+		list_del(&io_end_vec->list);
+		kmem_cache_free(io_end_vec_cachep, io_end_vec);
+	}
+}
+
+struct ext4_io_end_vec *ext4_last_io_end_vec(ext4_io_end_t *io_end)
+{
+	BUG_ON(list_empty(&io_end->list_vec));
+	return list_last_entry(&io_end->list_vec, struct ext4_io_end_vec, list);
+}
+
+/*
+ * Print an buffer I/O error compatible with the fs/buffer.c.  This
+ * provides compatibility with dmesg scrapers that look for a specific
+ * buffer I/O error message.  We really need a unified error reporting
+ * structure to userspace ala Digital Unix's uerf system, but it's
+ * probably not going to happen in my lifetime, due to LKML politics...
+ */
+static void buffer_io_error(struct buffer_head *bh)
+{
+	printk_ratelimited(KERN_ERR "Buffer I/O error on device %pg, logical block %llu\n",
+		       bh->b_bdev,
+			(unsigned long long)bh->b_blocknr);
+}
+
+static void ext4_finish_bio(struct bio *bio)
+{
+	struct folio_iter fi;
+
+	bio_for_each_folio_all(fi, bio) {
+		struct folio *folio = fi.folio;
+		struct buffer_head *bh, *head;
+		size_t bio_start = fi.offset;
+		size_t bio_end = bio_start + fi.length;
+		unsigned under_io = 0;
+		unsigned long flags;
+
+		if (bio->bi_status) {
+			int err = blk_status_to_errno(bio->bi_status);
+			mapping_set_error(folio->mapping, err);
+		}
+		bh = head = folio_buffers(folio);
+		/*
+		 * We check all buffers in the folio under b_uptodate_lock
+		 * to avoid races with other end io clearing async_write flags
+		 */
+		spin_lock_irqsave(&head->b_uptodate_lock, flags);
+		do {
+			if (bh_offset(bh) < bio_start ||
+			    bh_offset(bh) + bh->b_size > bio_end) {
+				if (buffer_async_write(bh))
+					under_io++;
+				continue;
+			}
+			clear_buffer_async_write(bh);
+			if (bio->bi_status) {
+				set_buffer_write_io_error(bh);
+				buffer_io_error(bh);
+			}
+		} while ((bh = bh->b_this_page) != head);
+		spin_unlock_irqrestore(&head->b_uptodate_lock, flags);
+		if (!under_io)
+			folio_end_writeback(folio);
+	}
+}
+
+static void ext4_release_io_end(ext4_io_end_t *io_end)
+{
+	struct bio *bio, *next_bio;
+
+	BUG_ON(!list_empty(&io_end->list));
+	BUG_ON(io_end->flag & EXT4_IO_END_UNWRITTEN);
+	WARN_ON(io_end->handle);
+
+	for (bio = io_end->bio; bio; bio = next_bio) {
+		next_bio = bio->bi_private;
+		ext4_finish_bio(bio);
+		bio_put(bio);
+	}
+	ext4_free_io_end_vec(io_end);
+	kmem_cache_free(io_end_cachep, io_end);
+}
+
+/*
+ * On successful IO, check a range of space and convert unwritten extents to
+ * written. On IO failure, check if journal abort is needed. Note that
+ * we are protected from truncate touching same part of extent tree by the
+ * fact that truncate code waits for all DIO to finish (thus exclusion from
+ * direct IO is achieved) and also waits for writeback to complete. Thus we
+ * cannot get to ext4_ext_truncate() before all IOs overlapping that range are
+ * completed (happens from ext4_free_ioend()).
+ */
+static int ext4_end_io_end(ext4_io_end_t *io_end)
+{
+	struct inode *inode = io_end->inode;
+	handle_t *handle = io_end->handle;
+	struct super_block *sb = inode->i_sb;
+	int ret = 0;
+
+	ext4_debug("ext4_end_io_nolock: io_end 0x%p from inode %llu,list->next 0x%p,"
+		   "list->prev 0x%p\n",
+		   io_end, inode->i_ino, io_end->list.next, io_end->list.prev);
+
+	/*
+	 * Do not convert the unwritten extents if data writeback fails,
+	 * or stale data may be exposed.
+	 */
+	io_end->handle = NULL;  /* Following call will use up the handle */
+	if (unlikely(io_end->flag & EXT4_IO_END_FAILED)) {
+		ret = -EIO;
+		if (handle)
+			jbd2_journal_free_reserved(handle);
+
+		if (test_opt(sb, DATA_ERR_ABORT))
+			jbd2_journal_abort(EXT4_SB(sb)->s_journal, ret);
+	} else {
+		ret = ext4_convert_unwritten_io_end_vec(handle, io_end);
+	}
+	if (ret < 0 && !ext4_emergency_state(sb) &&
+	    io_end->flag & EXT4_IO_END_UNWRITTEN) {
+		ext4_msg(sb, KERN_EMERG,
+			 "failed to convert unwritten extents to written "
+			 "extents -- potential data loss!  "
+			 "(inode %llu, error %d)", inode->i_ino, ret);
+	}
+
+	ext4_clear_io_unwritten_flag(io_end);
+	ext4_release_io_end(io_end);
+	return ret;
+}
+
+static void dump_completed_IO(struct inode *inode, struct list_head *head)
+{
+#ifdef	EXT4FS_DEBUG
+	struct list_head *cur, *before, *after;
+	ext4_io_end_t *io_end, *io_end0, *io_end1;
+
+	if (list_empty(head))
+		return;
+
+	ext4_debug("Dump inode %llu completed io list\n", inode->i_ino);
+	list_for_each_entry(io_end, head, list) {
+		cur = &io_end->list;
+		before = cur->prev;
+		io_end0 = container_of(before, ext4_io_end_t, list);
+		after = cur->next;
+		io_end1 = container_of(after, ext4_io_end_t, list);
+
+		ext4_debug("io 0x%p from inode %llu,prev 0x%p,next 0x%p\n",
+			    io_end, inode->i_ino, io_end0, io_end1);
+	}
+#endif
+}
+
+static bool ext4_io_end_defer_completion(ext4_io_end_t *io_end)
+{
+	if (io_end->flag & EXT4_IO_END_UNWRITTEN &&
+	    !list_empty(&io_end->list_vec))
+		return true;
+	if (test_opt(io_end->inode->i_sb, DATA_ERR_ABORT) &&
+	    io_end->flag & EXT4_IO_END_FAILED &&
+	    !ext4_emergency_state(io_end->inode->i_sb))
+		return true;
+	return false;
+}
+
+/* Add the io_end to per-inode completed end_io list. */
+static void ext4_add_complete_io(ext4_io_end_t *io_end)
+{
+	struct ext4_inode_info *ei = EXT4_I(io_end->inode);
+	struct ext4_sb_info *sbi = EXT4_SB(io_end->inode->i_sb);
+	struct workqueue_struct *wq;
+	unsigned long flags;
+
+	/* Only reserved conversions or pending IO errors will enter here. */
+	WARN_ON(!(io_end->flag & EXT4_IO_END_DEFER_COMPLETION));
+	WARN_ON(io_end->flag & EXT4_IO_END_UNWRITTEN &&
+		!io_end->handle && sbi->s_journal);
+	WARN_ON(!io_end->bio);
+
+	spin_lock_irqsave(&ei->i_completed_io_lock, flags);
+	wq = sbi->rsv_conversion_wq;
+	if (list_empty(&ei->i_rsv_conversion_list))
+		queue_work(wq, &ei->i_rsv_conversion_work);
+	list_add_tail(&io_end->list, &ei->i_rsv_conversion_list);
+	spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
+}
+
+static int ext4_do_flush_completed_IO(struct inode *inode,
+				      struct list_head *head)
+{
+	ext4_io_end_t *io_end;
+	struct list_head unwritten;
+	unsigned long flags;
+	struct ext4_inode_info *ei = EXT4_I(inode);
+	int err, ret = 0;
+
+	spin_lock_irqsave(&ei->i_completed_io_lock, flags);
+	dump_completed_IO(inode, head);
+	list_replace_init(head, &unwritten);
+	spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
+
+	while (!list_empty(&unwritten)) {
+		io_end = list_entry(unwritten.next, ext4_io_end_t, list);
+		BUG_ON(!(io_end->flag & EXT4_IO_END_DEFER_COMPLETION));
+		list_del_init(&io_end->list);
+
+		err = ext4_end_io_end(io_end);
+		if (unlikely(!ret && err))
+			ret = err;
+	}
+	return ret;
+}
+
+/*
+ * Used to convert unwritten extents to written extents upon IO completion,
+ * or used to abort the journal upon IO errors.
+ */
+void ext4_end_io_rsv_work(struct work_struct *work)
+{
+	struct ext4_inode_info *ei = container_of(work, struct ext4_inode_info,
+						  i_rsv_conversion_work);
+	ext4_do_flush_completed_IO(&ei->vfs_inode, &ei->i_rsv_conversion_list);
+}
+
+ext4_io_end_t *ext4_init_io_end(struct inode *inode, gfp_t flags)
+{
+	ext4_io_end_t *io_end = kmem_cache_zalloc(io_end_cachep, flags);
+
+	if (io_end) {
+		io_end->inode = inode;
+		INIT_LIST_HEAD(&io_end->list);
+		INIT_LIST_HEAD(&io_end->list_vec);
+		refcount_set(&io_end->count, 1);
+	}
+	return io_end;
+}
+
+void ext4_put_io_end_defer(ext4_io_end_t *io_end)
+{
+	if (refcount_dec_and_test(&io_end->count)) {
+		if (ext4_io_end_defer_completion(io_end))
+			return ext4_add_complete_io(io_end);
+
+		ext4_release_io_end(io_end);
+	}
+}
+
+int ext4_put_io_end(ext4_io_end_t *io_end)
+{
+	if (refcount_dec_and_test(&io_end->count)) {
+		if (ext4_io_end_defer_completion(io_end))
+			return ext4_end_io_end(io_end);
+
+		ext4_release_io_end(io_end);
+	}
+	return 0;
+}
+
+ext4_io_end_t *ext4_get_io_end(ext4_io_end_t *io_end)
+{
+	refcount_inc(&io_end->count);
+	return io_end;
+}
+
+/* BIO completion function for page writeback */
+static void ext4_end_bio(struct bio *bio)
+{
+	ext4_io_end_t *io_end = bio->bi_private;
+	sector_t bi_sector = bio->bi_iter.bi_sector;
+
+	if (WARN_ONCE(!io_end, "io_end is NULL: %pg: sector %Lu len %u err %d\n",
+		      bio->bi_bdev,
+		      (long long) bio->bi_iter.bi_sector,
+		      (unsigned) bio_sectors(bio),
+		      bio->bi_status)) {
+		ext4_finish_bio(bio);
+		bio_put(bio);
+		return;
+	}
+	bio->bi_end_io = NULL;
+
+	if (bio->bi_status) {
+		struct inode *inode = io_end->inode;
+
+		ext4_warning(inode->i_sb, "I/O error %d writing to inode %llu "
+			     "starting block %llu)",
+			     bio->bi_status, inode->i_ino,
+			     (unsigned long long)
+			     bi_sector >> (inode->i_blkbits - 9));
+		io_end->flag |= EXT4_IO_END_FAILED;
+		mapping_set_error(inode->i_mapping,
+				blk_status_to_errno(bio->bi_status));
+	}
+
+	if (ext4_io_end_defer_completion(io_end)) {
+		/*
+		 * Link bio into list hanging from io_end. We have to do it
+		 * atomically as bio completions can be racing against each
+		 * other.
+		 */
+		bio->bi_private = xchg(&io_end->bio, bio);
+		ext4_put_io_end_defer(io_end);
+	} else {
+		/*
+		 * Drop io_end reference early. Inode can get freed once
+		 * we finish the bio.
+		 */
+		ext4_put_io_end_defer(io_end);
+		ext4_finish_bio(bio);
+		bio_put(bio);
+	}
+}
+
+void ext4_io_submit(struct ext4_io_submit *io)
+{
+	struct bio *bio = io->io_bio;
+
+	if (bio) {
+		if (io->io_wbc->sync_mode == WB_SYNC_ALL)
+			io->io_bio->bi_opf |= REQ_SYNC;
+		blk_crypto_submit_bio(io->io_bio);
+	}
+	io->io_bio = NULL;
+}
+
+void ext4_io_submit_init(struct ext4_io_submit *io,
+			 struct writeback_control *wbc)
+{
+	io->io_wbc = wbc;
+	io->io_bio = NULL;
+	io->io_end = NULL;
+}
+
+static void io_submit_init_bio(struct ext4_io_submit *io,
+			       struct inode *inode,
+			       struct folio *folio,
+			       struct buffer_head *bh)
+{
+	struct bio *bio;
+
+	/*
+	 * bio_alloc will _always_ be able to allocate a bio if
+	 * __GFP_DIRECT_RECLAIM is set, see comments for bio_alloc_bioset().
+	 */
+	bio = bio_alloc(bh->b_bdev, BIO_MAX_VECS, REQ_OP_WRITE, GFP_NOIO);
+	fscrypt_set_bio_crypt_ctx(bio, inode, folio_pos(folio) + bh_offset(bh),
+				  GFP_NOIO);
+	bio->bi_iter.bi_sector = bh->b_blocknr * (bh->b_size >> 9);
+	bio->bi_end_io = ext4_end_bio;
+	bio->bi_private = ext4_get_io_end(io->io_end);
+	bio->bi_write_hint = inode->i_write_hint;
+	io->io_bio = bio;
+	io->io_next_block = bh->b_blocknr;
+	wbc_init_bio(io->io_wbc, bio);
+}
+
+static bool io_submit_need_new_bio(struct ext4_io_submit *io,
+				   struct inode *inode,
+				   struct folio *folio,
+				   struct buffer_head *bh)
+{
+	if (bh->b_blocknr != io->io_next_block)
+		return true;
+	if (!fscrypt_mergeable_bio(io->io_bio, inode,
+			folio_pos(folio) + bh_offset(bh)))
+		return true;
+	return false;
+}
+
+static void io_submit_add_bh(struct ext4_io_submit *io,
+			     struct inode *inode,
+			     struct folio *folio,
+			     struct buffer_head *bh)
+{
+	if (io->io_bio && io_submit_need_new_bio(io, inode, folio, bh)) {
+submit_and_retry:
+		ext4_io_submit(io);
+	}
+	if (io->io_bio == NULL)
+		io_submit_init_bio(io, inode, folio, bh);
+	if (!bio_add_folio(io->io_bio, folio, bh->b_size, bh_offset(bh)))
+		goto submit_and_retry;
+	wbc_account_cgroup_owner(io->io_wbc, folio, bh->b_size);
+	io->io_next_block++;
+}
+
+void ext4_bio_write_folio(struct ext4_io_submit *io, struct folio *folio,
+		size_t len)
+{
+	struct inode *inode = folio->mapping->host;
+	unsigned block_start;
+	struct buffer_head *bh, *head;
+	int nr_to_submit = 0;
+	struct writeback_control *wbc = io->io_wbc;
+	bool keep_towrite = false;
+
+	BUG_ON(!folio_test_locked(folio));
+	BUG_ON(folio_test_writeback(folio));
+
+	/*
+	 * Comments copied from block_write_full_folio:
+	 *
+	 * The folio straddles i_size.  It must be zeroed out on each and every
+	 * writepage invocation because it may be mmapped.  "A file is mapped
+	 * in multiples of the page size.  For a file that is not a multiple of
+	 * the page size, the remaining memory is zeroed when mapped, and
+	 * writes to that region are not written out to the file."
+	 */
+	if (len < folio_size(folio))
+		folio_zero_segment(folio, len, folio_size(folio));
+	/*
+	 * In the first loop we prepare and mark buffers to submit. We have to
+	 * mark all buffers in the folio before submitting so that
+	 * folio_end_writeback() cannot be called from ext4_end_bio() when IO
+	 * on the first buffer finishes and we are still working on submitting
+	 * the second buffer.
+	 */
+	bh = head = folio_buffers(folio);
+	do {
+		block_start = bh_offset(bh);
+		if (block_start >= len) {
+			clear_buffer_dirty(bh);
+			set_buffer_uptodate(bh);
+			continue;
+		}
+		if (!buffer_dirty(bh) || buffer_delay(bh) ||
+		    !buffer_mapped(bh) || buffer_unwritten(bh)) {
+			/* A hole? We can safely clear the dirty bit */
+			if (!buffer_mapped(bh))
+				clear_buffer_dirty(bh);
+			/*
+			 * Keeping dirty some buffer we cannot write? Make sure
+			 * to redirty the folio and keep TOWRITE tag so that
+			 * racing WB_SYNC_ALL writeback does not skip the folio.
+			 * This happens e.g. when doing writeout for
+			 * transaction commit or when journalled data is not
+			 * yet committed.
+			 */
+			if (buffer_dirty(bh) ||
+			    (buffer_jbd(bh) && buffer_jbddirty(bh))) {
+				if (!folio_test_dirty(folio))
+					folio_redirty_for_writepage(wbc, folio);
+				keep_towrite = true;
+			}
+			continue;
+		}
+		if (buffer_new(bh))
+			clear_buffer_new(bh);
+		set_buffer_async_write(bh);
+		clear_buffer_dirty(bh);
+		nr_to_submit++;
+	} while ((bh = bh->b_this_page) != head);
+
+	if (!nr_to_submit) {
+		/*
+		 * We have nothing to submit. Just cycle the folio through
+		 * writeback state to properly update xarray tags.
+		 */
+		__folio_start_writeback(folio, keep_towrite);
+		folio_end_writeback(folio);
+		return;
+	}
+
+	bh = head = folio_buffers(folio);
+
+	__folio_start_writeback(folio, keep_towrite);
+
+	/* Now submit buffers to write */
+	do {
+		if (!buffer_async_write(bh))
+			continue;
+		io_submit_add_bh(io, inode, folio, bh);
+	} while ((bh = bh->b_this_page) != head);
+}