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

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Message-Id: <20250915164248.788601c4dc614913081ec7d7@linux-foundation.org>
Date: Mon, 15 Sep 2025 16:42:48 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Donet Tom <donettom@...ux.ibm.com>
Cc: 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, 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?

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.

Maybe the -stable maintainers parse the "# v6.6" thing, I don't know. 
But providing them with a single Fixes: recommendation is a good thing
either way.

So can you please revisit this and suggest a single Fixes: target which
clarifies your reccomendation?

Thanks.

(Cc Joe.  Should checkpatch say something about this)?