linux-kernel - [PATCH 1/2] mm/slub: Fix cmp_loc_by_count() to return 0 when counts are equal

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Message-Id: <20250825013419.240278-2-visitorckw@gmail.com>
Date: Mon, 25 Aug 2025 09:34:18 +0800
From: Kuan-Wei Chiu <visitorckw@...il.com>
To: vbabka@...e.cz,
	akpm@...ux-foundation.org
Cc: cl@...two.org,
	rientjes@...gle.com,
	roman.gushchin@...ux.dev,
	harry.yoo@...cle.com,
	glittao@...il.com,
	jserv@...s.ncku.edu.tw,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Kuan-Wei Chiu <visitorckw@...il.com>,
	stable@...r.kernel.org
Subject: [PATCH 1/2] mm/slub: Fix cmp_loc_by_count() to return 0 when counts are equal

The comparison function cmp_loc_by_count() used for sorting stack trace
locations in debugfs currently returns -1 if a->count > b->count and 1
otherwise. This breaks the antisymmetry property required by sort(),
because when two counts are equal, both cmp(a, b) and cmp(b, a) return
1.

This can lead to undefined or incorrect ordering results. Fix it by
explicitly returning 0 when the counts are equal, ensuring that the
comparison function follows the expected mathematical properties.

Fixes: 553c0369b3e1 ("mm/slub: sort debugfs output by frequency of stack traces")
Cc: stable@...r.kernel.org
Signed-off-by: Kuan-Wei Chiu <visitorckw@...il.com>
---
 mm/slub.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index 30003763d224..c91b3744adbc 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -7718,8 +7718,9 @@ static int cmp_loc_by_count(const void *a, const void *b, const void *data)
 
 	if (loc1->count > loc2->count)
 		return -1;
-	else
+	if (loc1->count < loc2->count)
 		return 1;
+	return 0;
 }
 
 static void *slab_debugfs_start(struct seq_file *seq, loff_t *ppos)
-- 
2.34.1