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

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Message-Id: <20250915214117.5117d339669e091b1d3fa96d@linux-foundation.org>
Date: Mon, 15 Sep 2025 21:41:17 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Sasha Levin <sashal@...nel.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 Tue, 16 Sep 2025 00:33:09 -0400 Sasha Levin <sashal@...nel.org> wrote:

> 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.

OK.

> 
> >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.

So what should we do in this situation?