[PATCH] mm/filemap: fix page_cache_prev_miss() when no hole is found

[Tal Zussman]

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.

Mirror the fix in commit bbcaee20e03e ("readahead: fix return value of
page_cache_next_miss() when no hole is found"): preserve max_scan in a
separate variable across the loop and return `index - max_scan` from the
no-gap-found path.

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.

Fixes: 0d3f92966629 ("page cache: Convert hole search to XArray")
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Tal Zussman <tz2294@xxxxxxxxxxxx>
---
 mm/filemap.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index ab34cab2416a..545ccd59777d 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1845,16 +1845,17 @@ pgoff_t page_cache_prev_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 (max_scan--) {
+	while (nr--) {
 		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 index - max_scan;
 }
 EXPORT_SYMBOL(page_cache_prev_miss);
 

---
base-commit: e9dd96806dbc2d50a66770b6a86962bd5d601153
change-id: 20260510-prev_miss_fix-fcb308472131

Best regards,
-- 
Tal Zussman <tz2294@xxxxxxxxxxxx>