linux-kernel - Re: [PATCH v2 1/3] mm/ksm: Fix incorrect KSM counter handling in mm

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Message-ID: <aMjohar0r-nffx9V@laps>
Date: Tue, 16 Sep 2025 00:33:09 -0400
From: Sasha Levin <sashal@...nel.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Donet Tom <donettom@...ux.ibm.com>,
	David Hildenbrand <david@...hat.com>,
	Ritesh Harjani <ritesh.list@...il.com>,
	Xu Xin <xu.xin16@....com.cn>,
	Chengming Zhou <chengming.zhou@...ux.dev>,
	Wei Yang <richard.weiyang@...il.com>,
	Aboorva Devarajan <aboorvad@...ux.ibm.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Giorgi Tchankvetadze <giorgitchankvetadze1997@...il.com>,
	stable@...r.kernel.org, Joe Perches <joe@...ches.com>
Subject: Re: [PATCH v2 1/3] mm/ksm: Fix incorrect KSM counter handling in
 mm_struct during fork

On Mon, Sep 15, 2025 at 04:42:48PM -0700, Andrew Morton wrote:
>On Mon, 15 Sep 2025 20:33:04 +0530 Donet Tom <donettom@...ux.ibm.com> wrote:
>
>> Currently, the KSM-related counters in `mm_struct`, such as
>> `ksm_merging_pages`, `ksm_rmap_items`, and `ksm_zero_pages`, are
>> inherited by the child process during fork. This results in inconsistent
>> accounting.
>>
>> When a process uses KSM, identical pages are merged and an rmap item is
>> created for each merged page. The `ksm_merging_pages` and
>> `ksm_rmap_items` counters are updated accordingly. However, after a
>> fork, these counters are copied to the child while the corresponding
>> rmap items are not. As a result, when the child later triggers an
>> unmerge, there are no rmap items present in the child, so the counters
>> remain stale, leading to incorrect accounting.
>>
>> A similar issue exists with `ksm_zero_pages`, which maintains both a
>> global counter and a per-process counter. During fork, the per-process
>> counter is inherited by the child, but the global counter is not
>> incremented. Since the child also references zero pages, the global
>> counter should be updated as well. Otherwise, during zero-page unmerge,
>> both the global and per-process counters are decremented, causing the
>> global counter to become inconsistent.
>>
>> To fix this, ksm_merging_pages and ksm_rmap_items are reset to 0
>> during fork, and the global ksm_zero_pages counter is updated with the
>> per-process ksm_zero_pages value inherited by the child. This ensures
>> that KSM statistics remain accurate and reflect the activity of each
>> process correctly.
>>
>> Fixes: 7609385337a4 ("ksm: count ksm merging pages for each process")
>
>Linux-v5.19
>
>> Fixes: cb4df4cae4f2 ("ksm: count allocated ksm rmap_items for each process")
>
>Linux-v6.1
>
>> Fixes: e2942062e01d ("ksm: count all zero pages placed by KSM")
>
>Linux-v6.10
>
>> cc: stable@...r.kernel.org # v6.6
>
>So how was Linux-v6.6 arrived at?

e2942062e01d is in v6.6, not in v6.10 - I suspect that this is why the "# v6.6"
part was added.

>I think the most important use for Fixes: is to tell the -stable
>maintainers which kernel version(s) we believe should receive the
>patch.  So listing multiple Fixes: targets just causes confusion.

Right - there's no way of communicating if all the commits listed in multiple
Fixes tags should exist in the tree, or any one of them, for the new fix to be
applicable.

-- 
Thanks,
Sasha