netfs: zero the tail of a short DIO/unbuffered readgrafted

The buffered read collector zero-fills the tail of a short read that
stops below the inode's i_size (netfs_clear_unread()), so a read that
races an extending write still returns the expected number of bytes.
The non-buffered collector path does no such thing: it just records
how much was transferred.

Add the same zero-fill for the non-buffered case, gated on
NETFS_SREQ_CLEAR_TAIL: a subreq's source sets that flag when a short
result from it is known to be safe to treat as a hole, as opposed to
NETFS_SREQ_HIT_EOF, which means the read genuinely ran off the end of
the file and should be reported short as-is. Only CLEAR_TAIL should
zero-fill here; a real EOF must stay a real short read.

No source currently sets CLEAR_TAIL on an unbuffered/DIO subrequest,
so this is inert on its own -- a following change teaches cifs to set
it in the one case that needs it.

Fixes: e2d46f2ec332 ("netfs: Change the read result collector to only use one work item")
Reviewed-by: David Howells <dhowells@redhat.com>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Shyam Prasad N <sprasad@microsoft.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Bharath SM <bharathsm@microsoft.com>
Cc: stable@vger.kernel.org

1 files changed, 73 insertions, 0 deletions

diff --git a/include/asm-generic/pgtable-nopmd.h b/include/asm-generic/pgtable-nopmd.h
new file mode 100644
index 000000000..36b6490ed
--- /dev/null
+++ b/include/asm-generic/pgtable-nopmd.h
@@ -0,0 +1,73 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _PGTABLE_NOPMD_H
+#define _PGTABLE_NOPMD_H
+
+#ifndef __ASSEMBLER__
+
+#include <asm-generic/pgtable-nopud.h>
+
+struct mm_struct;
+
+#define __PAGETABLE_PMD_FOLDED 1
+
+/*
+ * Having the pmd type consist of a pud gets the size right, and allows
+ * us to conceptually access the pud entry that this pmd is folded into
+ * without casting.
+ */
+typedef struct { pud_t pud; } pmd_t;
+
+#define PMD_SHIFT	PUD_SHIFT
+#define PTRS_PER_PMD	1
+#define PMD_SIZE  	(1UL << PMD_SHIFT)
+#define PMD_MASK  	(~(PMD_SIZE-1))
+
+/*
+ * The "pud_xxx()" functions here are trivial for a folded two-level
+ * setup: the pmd is never bad, and a pmd always exists (as it's folded
+ * into the pud entry)
+ */
+static inline int pud_none(pud_t pud)		{ return 0; }
+static inline int pud_bad(pud_t pud)		{ return 0; }
+static inline int pud_present(pud_t pud)	{ return 1; }
+static inline int pud_user(pud_t pud)		{ return 0; }
+static inline int pud_leaf(pud_t pud)		{ return 0; }
+static inline void pud_clear(pud_t *pud)	{ }
+#define pmd_ERROR(pmd)				(pud_ERROR((pmd).pud))
+
+#define pud_populate(mm, pmd, pte)		do { } while (0)
+
+/*
+ * (pmds are folded into puds so this doesn't get actually called,
+ * but the define is needed for a generic inline function.)
+ */
+#define set_pud(pudptr, pudval)			set_pmd((pmd_t *)(pudptr), (pmd_t) { pudval })
+
+static inline pmd_t * pmd_offset(pud_t * pud, unsigned long address)
+{
+	return (pmd_t *)pud;
+}
+#define pmd_offset pmd_offset
+
+#define pmd_val(x)				(pud_val((x).pud))
+#define __pmd(x)				((pmd_t) { __pud(x) } )
+
+#define pud_page(pud)				(pmd_page((pmd_t){ pud }))
+#define pud_pgtable(pud)			((pmd_t *)(pmd_page_vaddr((pmd_t){ pud })))
+
+/*
+ * allocating and freeing a pmd is trivial: the 1-entry pmd is
+ * inside the pud, so has no extra memory associated with it.
+ */
+#define pmd_alloc_one(mm, address)		NULL
+static inline void pmd_free(struct mm_struct *mm, pmd_t *pmd)
+{
+}
+#define pmd_free_tlb(tlb, x, a)		do { } while (0)
+
+#undef  pmd_addr_end
+#define pmd_addr_end(addr, end)			(end)
+
+#endif /* __ASSEMBLER__ */
+
+#endif /* _PGTABLE_NOPMD_H */