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

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Message-ID: <aK1n_t-V1AlN86JR@hyeyoo>
Date: Tue, 26 Aug 2025 16:53:34 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Kuan-Wei Chiu <visitorckw@...il.com>
Cc: Vlastimil Babka <vbabka@...e.cz>, akpm@...ux-foundation.org, cl@...two.org,
        rientjes@...gle.com, roman.gushchin@...ux.dev, glittao@...il.com,
        jserv@...s.ncku.edu.tw, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Joshua Hahn <joshua.hahnjy@...il.com>
Subject: Re: [PATCH 1/2] mm/slub: Fix cmp_loc_by_count() to return 0 when
 counts are equal

On Tue, Aug 26, 2025 at 01:54:49AM +0800, Kuan-Wei Chiu wrote:
> 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.

Could you give an example where the previous cmp_loc_by_count() code
produces an incorrectly sorted array?

> [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.

-- 
Cheers,
Harry / Hyeonggon