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

[Kuan-Wei Chiu <visitorckw@gmail.com>]

Hi Vlastimil,

On Mon, Aug 25, 2025 at 07:28:17PM +0200, Vlastimil Babka wrote:
> On 8/25/25 03:34, 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.
> 
> Good catch.
> 
> > This can lead to undefined or incorrect ordering results. Fix it by
> 
> Wonder if it can really affect anything in practice other than swapping
> needlessly some records with an equal count?
> 
It could result in some elements being incorrectly ordered, similar to
what happened before in ACPI causing issues with s2idle [1][2]. But in
this case, the worst impact is just the display order not matching the
count, so it's not too critical.

[1]: https://lore.kernel.org/lkml/70674dc7-5586-4183-8953-8095567e73df@gmail.com/
[2]: https://lore.kernel.org/lkml/20240701205639.117194-1-visitorckw@gmail.com/

> > explicitly returning 0 when the counts are equal, ensuring that the
> > comparison function follows the expected mathematical properties.
> 
> Agreed with the cmp_int() suggestion for a v2.
> 
I'll make that change in v2.

> > Fixes: 553c0369b3e1 ("mm/slub: sort debugfs output by frequency of stack traces")
> > Cc: stable@vger.kernel.org
> 
> I don't think it can cause any serious bugs so Cc: stable is unnecessary.
> 
I'll drop it in v2.

Regards,
Kuan-Wei

> > Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
> 
> Thanks!
> 
> > ---
> >  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)
>