[PATCH v2 0/2] mm/slub: Fix debugfs stack trace sorting and simplify sort call

[PATCH v2 0/2] mm/slub: Fix debugfs stack trace sorting and simplify sort call

[Kuan-Wei Chiu]

Fix the comparison function used for sorting stack trace locations in
the slub debugfs interface. The original implementation violated the
antisymmetry property required by sort(), which could lead to
unreliable ordering of the output. The patches correct the comparison
function to return 0 when counts are equal and replace the unnecessary
use of sort_r() with the simpler sort().
---
Changes in v2:
* Use cmp_int().
* Drop Cc stable. 

v1: https://lore.kernel.org/lkml/20250825013419.240278-1-visitorckw@gmail.com/

Kuan-Wei Chiu (2):
  mm/slub: Fix cmp_loc_by_count() to return 0 when counts are equal
  mm/slub: Replace sort_r() with sort() for debugfs stack trace sorting

 mm/slub.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

-- 
2.34.1

[PATCH v2 1/2] mm/slub: Fix cmp_loc_by_count() to return 0 when counts are equal

[Kuan-Wei Chiu]

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

[PATCH v2 2/2] mm/slub: Replace sort_r() with sort() for debugfs stack trace sorting

[Kuan-Wei Chiu]

The comparison function used to sort stack trace locations in debugfs
never relied on the third argument. Therefore, sort_r() is unnecessary.
Switch to sort() with a two-argument comparison function to keep the
code simple and aligned with the intended usage.

Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
---
 mm/slub.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 081816ff89ab..39a238384892 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -7711,7 +7711,7 @@ static void *slab_debugfs_next(struct seq_file *seq, void *v, loff_t *ppos)
 	return NULL;
 }
 
-static int cmp_loc_by_count(const void *a, const void *b, const void *data)
+static int cmp_loc_by_count(const void *a, const void *b)
 {
 	struct location *loc1 = (struct location *)a;
 	struct location *loc2 = (struct location *)b;
@@ -7778,8 +7778,8 @@ static int slab_debug_trace_open(struct inode *inode, struct file *filep)
 	}
 
 	/* Sort locations by count */
-	sort_r(t->loc, t->count, sizeof(struct location),
-		cmp_loc_by_count, NULL, NULL);
+	sort(t->loc, t->count, sizeof(struct location),
+	     cmp_loc_by_count, NULL);
 
 	bitmap_free(obj_map);
 	return 0;
-- 
2.34.1

Re: [PATCH v2 0/2] mm/slub: Fix debugfs stack trace sorting and simplify sort call

[Vlastimil Babka]

On 8/26/25 08:23, Kuan-Wei Chiu wrote:
> Fix the comparison function used for sorting stack trace locations in
> the slub debugfs interface. The original implementation violated the
> antisymmetry property required by sort(), which could lead to
> unreliable ordering of the output. The patches correct the comparison
> function to return 0 when counts are equal and replace the unnecessary
> use of sort_r() with the simpler sort().
> ---
> Changes in v2:
> * Use cmp_int().
> * Drop Cc stable. 
> 
> v1: https://lore.kernel.org/lkml/20250825013419.240278-1-visitorckw@gmail.com/
> 
> Kuan-Wei Chiu (2):
>   mm/slub: Fix cmp_loc_by_count() to return 0 when counts are equal
>   mm/slub: Replace sort_r() with sort() for debugfs stack trace sorting

Applied to slab/for-next, thanks!

>  mm/slub.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
> 

Re: [PATCH v2 1/2] mm/slub: Fix cmp_loc_by_count() to return 0 when counts are equal

[Harry Yoo]

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

Re: [PATCH v2 2/2] mm/slub: Replace sort_r() with sort() for debugfs stack trace sorting

[Harry Yoo]

On Tue, Aug 26, 2025 at 02:23:15PM +0800, Kuan-Wei Chiu wrote:
> The comparison function used to sort stack trace locations in debugfs
> never relied on the third argument. Therefore, sort_r() is unnecessary.
> Switch to sort() with a two-argument comparison function to keep the
> code simple and aligned with the intended usage.
> 
> Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
> ---

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

>  mm/slub.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 081816ff89ab..39a238384892 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -7711,7 +7711,7 @@ static void *slab_debugfs_next(struct seq_file *seq, void *v, loff_t *ppos)
>  	return NULL;
>  }
>  
> -static int cmp_loc_by_count(const void *a, const void *b, const void *data)
> +static int cmp_loc_by_count(const void *a, const void *b)
>  {
>  	struct location *loc1 = (struct location *)a;
>  	struct location *loc2 = (struct location *)b;
> @@ -7778,8 +7778,8 @@ static int slab_debug_trace_open(struct inode *inode, struct file *filep)
>  	}
>  
>  	/* Sort locations by count */
> -	sort_r(t->loc, t->count, sizeof(struct location),
> -		cmp_loc_by_count, NULL, NULL);
> +	sort(t->loc, t->count, sizeof(struct location),
> +	     cmp_loc_by_count, NULL);
>  
>  	bitmap_free(obj_map);
>  	return 0;
> -- 
> 2.34.1
> 

-- 
Cheers,
Harry / Hyeonggon