From: Chi Zhiling <chizhiling@kylinos.cn>
Date: Thu, 5 Jun 2025 05:49:35 +0000 (+0800)
Subject: readahead: fix return value of page_cache_next_miss() when no hole is found
X-Git-Tag: v6.12.44~250
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=72e849b5b16a48ac6dc6c09997ce38b90b0f7d86;p=thirdparty%2Fkernel%2Fstable.git

readahead: fix return value of page_cache_next_miss() when no hole is found

commit bbcaee20e03ecaeeecba32a703816a0d4502b6c4 upstream.

max_scan in page_cache_next_miss always decreases to zero when no hole is
found, causing the return value to be index + 0.

Fix this by preserving the max_scan value throughout the loop.

Jan said "From what I know and have seen in the past, wrong responses
from page_cache_next_miss() can lead to readahead window reduction and
thus reduced read speeds."

Link: https://lkml.kernel.org/r/20250605054935.2323451-1-chizhiling@163.com
Fixes: 901a269ff3d5 ("filemap: fix page_cache_next_miss() when no hole found")
Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---

diff --git a/mm/filemap.c b/mm/filemap.c
index fa18e71f9c889..ec69fadf014cd 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1750,8 +1750,9 @@ 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 (max_scan--) {
+	while (nr--) {
 		void *entry = xas_next(&xas);
 		if (!entry || xa_is_value(entry))
 			return xas.xa_index;