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>

0 files changed, 0 insertions, 0 deletions