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>

diff --git a/mm/filemap.c b/mm/filemap.c
index ab34cab2416a45889b9aa026ca8d365b394f176e..4263d9775998520afeff8658615a63279d32d4de 100644 (file)
--- 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);