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: <aLELGQDzpjmQ4ppP@hyeyoo>
Date: Fri, 29 Aug 2025 11:06:17 +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>, chuang@...nycu.edu.tw,
        cfmc.cs13@...u.edu.tw, jhcheng.cs13@...u.edu.tw, c.yuanhaur@...tl.edu
Subject: Re: [PATCH 1/2] mm/slub: Fix cmp_loc_by_count() to return 0 when
 counts are equal

On Fri, Aug 29, 2025 at 01:13:58AM +0800, Kuan-Wei Chiu wrote:
> On Tue, Aug 26, 2025 at 04:53:34PM +0900, Harry Yoo wrote:
> > 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?
> > 
> Sorry for the late reply.

No problem ;)

> I tried generating random arrays to find a concrete example where the
> old cmp_loc_by_count() causes a wrong ordering, but I couldn't
> reproduce one. So I would like to withdraw my earlier claim that it
> definitely leads to incorrect results, since I cannot demonstrate a
> failing case.

Yeah I couldn't either. Maybe mathematical proof would work, but I
didn't try.

> That said, I still believe the patch should be merged, because sort()
> only guarantees correct behavior if the comparison function satisfies
> antisymmetry and transitivity. When those are violated, correctness
> depends on implementation details, and future changes (e.g., switching
> to a different sorting algorithm) could potentially break the ordering.

Agreed. No doubt the series is worth merging, just wanted to clarify
that bit.

Thanks!

-- 
Cheers,
Harry / Hyeonggon

> Regards,
> Kuan-Wei
> 
> > > [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.