exfat: fix valid_size extension over a shared writable mapping

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When a shared writable mapping has its valid_size extended by a buffered
write or a page fault, exfat zeroes the page-cache gap below the new
valid_size. A store through the mapping can race with this zeroing and be
overwritten.

Fix this by zeroing the gap lazily. Drop ->map_pages so that every first
write fault goes through exfat_page_mkwrite(), which advances valid_size to
cover the faulting page. With fault-around enabled, a store could install a
writable PTE, skip ->page_mkwrite(), and land past valid_size without
advancing it. Extending valid_size one faulting page at a time also leaves
never-written pages in a large mapping alone.

The gap is filled with block granularity, zeroing only the not-uptodate
blocks and preserving blocks that may hold data stored through the mapping.
On the buffered-write path the invalidate lock is held and the gap is
unmapped before zeroing, so a racing store re-faults and, under the inode
lock, completes only after the gap has been zeroed and valid_size covers
it.

Fixes: 82a81a7352 ("exfat: add iomap buffered I/O support")
Co-developed-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

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This commit is contained in:

Namjae Jeon 2026-07-22 14:14:11 +09:00

parent 1a3746ccbb

commit 1135704ed2

3 changed files with 153 additions and 40 deletions

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2

fs/exfat/exfat_fs.h

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180

fs/exfat/file.c

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11

fs/exfat/iomap.c

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