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
updating the comparison logic to explicitly handle the case when counts
are equal, and use cmp_int() to ensure the comparison function adheres
to the required mathematical properties of antisymmetry.

Fixes: 553c0369b3e1 ("mm/slub: sort debugfs output by frequency of stack traces")
Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
---
 mm/slub.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 30003763d224..081816ff89ab 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -7716,10 +7716,7 @@ static int cmp_loc_by_count(const void *a, const void *b, const void *data)
 	struct location *loc1 = (struct location *)a;
 	struct location *loc2 = (struct location *)b;
 
-	if (loc1->count > loc2->count)
-		return -1;
-	else
-		return 1;
+	return cmp_int(loc2->count, loc1->count);
 }
 
 static void *slab_debugfs_start(struct seq_file *seq, loff_t *ppos)
-- 
2.34.1

On Tue, Aug 26, 2025 at 02:23:14PM +0800, Kuan-Wei Chiu wrote:
> 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
> updating the comparison logic to explicitly handle the case when counts
> are equal, and use cmp_int() to ensure the comparison function adheres
> to the required mathematical properties of antisymmetry.
> 
> Fixes: 553c0369b3e1 ("mm/slub: sort debugfs output by frequency of stack traces")
> Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>
> Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
> ---

While the author withdrew the claim that it definitely leads to incorrect
results, it remains true that the API requires both transitivity and
antisymmetry for correctness, so:

Reviewed-by: Harry Yoo <harry.yoo@oracle.com>

-- 
Cheers,
Harry / Hyeonggon

>  mm/slub.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 30003763d224..081816ff89ab 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -7716,10 +7716,7 @@ static int cmp_loc_by_count(const void *a, const void *b, const void *data)
>  	struct location *loc1 = (struct location *)a;
>  	struct location *loc2 = (struct location *)b;
>  
> -	if (loc1->count > loc2->count)
> -		return -1;
> -	else
> -		return 1;
> +	return cmp_int(loc2->count, loc1->count);
>  }
>  
>  static void *slab_debugfs_start(struct seq_file *seq, loff_t *ppos)
> -- 
> 2.34.1