diff options
| author | Tal Zussman <tz2294@columbia.edu> | 2026-05-12 16:45:59 -0400 |
|---|---|---|
| committer | Andrew Morton <akpm@linux-foundation.org> | 2026-05-28 21:31:04 -0700 |
| commit | cd0392875ce031dd5e01620798e70897490bac6a (patch) | |
| tree | 05f6b601ad7eba5e87a3e961dd3cc796128cf359 /mm | |
| parent | 0cc6f673d636a5b521971998af0f89d95943ffb4 (diff) | |
| download | linux-next-history-cd0392875ce031dd5e01620798e70897490bac6a.tar.gz | |
mm/filemap: fix page_cache_prev_miss() when no hole is found
page_cache_prev_miss() is documented to return a value outside the
searched range when no gap is found. However, the no-gap-found path
returns xas.xa_index, which after a successful loop is the first index in
the range. As such, that index is misreported as a gap.
The sole caller, page_cache_sync_ra(), uses the return value to estimate
the cached run preceding a sequential read. In some cases, the buggy
return value can undercount the contiguous range by one, shrinking the
readahead window or pushing borderline requests into the small-random-read
branch.
Fix this by returning the start of the range - 1 when no hole is found.
Update page_cache_next_miss() for clarity as well.
Both helpers were previously fixed together in commit 9425c591e06a ("page
cache: fix page_cache_next/prev_miss off by one"), but the fix was
reverted because it caused a hugetlb performance regression. hugetlb no
longer uses these functions and next_miss was subsequently refixed in
commit 901a269ff3d5 ("filemap: fix page_cache_next_miss() when no hole
found") and commit bbcaee20e03e ("readahead: fix return value of
page_cache_next_miss() when no hole is found"), but prev_miss was not
addressed.
This was found by pointing Claude Opus 4.7 at mm/filemap.c.
Link: https://lore.kernel.org/20260512-prev_miss_fix-v2-1-4af8e5c1ae62@columbia.edu
Fixes: 0d3f92966629 ("page cache: Convert hole search to XArray")
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Tal Zussman <tz2294@columbia.edu>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Vishal Moola <vishal.moola@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Diffstat (limited to 'mm')
| -rw-r--r-- | mm/filemap.c | 13 |
1 files changed, 7 insertions, 6 deletions
diff --git a/mm/filemap.c b/mm/filemap.c index ab34cab2416a4..4263d97759985 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -1808,9 +1808,8 @@ pgoff_t page_cache_next_miss(struct address_space *mapping, pgoff_t index, unsigned long max_scan) { XA_STATE(xas, &mapping->i_pages, index); - unsigned long nr = max_scan; - while (nr--) { + while (max_scan--) { void *entry = xas_next(&xas); if (!entry || xa_is_value(entry)) return xas.xa_index; @@ -1818,7 +1817,8 @@ pgoff_t page_cache_next_miss(struct address_space *mapping, return 0; } - return index + max_scan; + /* Return end of the range + 1 when no hole is found */ + return xas.xa_index + 1; } EXPORT_SYMBOL(page_cache_next_miss); @@ -1849,12 +1849,13 @@ pgoff_t page_cache_prev_miss(struct address_space *mapping, while (max_scan--) { void *entry = xas_prev(&xas); if (!entry || xa_is_value(entry)) - break; + return xas.xa_index; if (xas.xa_index == ULONG_MAX) - break; + return ULONG_MAX; } - return xas.xa_index; + /* Return start of the range - 1 when no hole is found */ + return xas.xa_index - 1; } EXPORT_SYMBOL(page_cache_prev_miss); |