Re: [PATCH v2 1/3] mm/ksm: Fix incorrect KSM counter handling in mm_struct during fork

[Sasha Levin]

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@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx # 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