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 --- 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