Re: [PATCH] mm/vmscan: respect mems_effective in demote_folio_list()

[Bing Jiao <bingjiao@google.com>]

On Sat, Dec 20, 2025 at 11:20:44AM -0800, Andrew Morton wrote:
> On Sat, 20 Dec 2025 06:10:21 +0000 Bing Jiao <bingjiao@google.com> wrote:
>
> > Commit 7d709f49babc ("vmscan,cgroup: apply mems_effective to reclaim")
> > introduces the cpuset.mems_effective check and applies it to
> > can_demote().
>
> So we'll want
>
> 	Fixes: 7d709f49babc ("vmscan,cgroup: apply mems_effective to reclaim")
>
> in the changelog.
>
> > However, it does not apply this check in
> > demote_folio_list(), which leads to situations where pages are demoted
> > to nodes that are explicitly excluded from the task's cpuset.mems.
> >
> > To address the issue that demotion targets do not respect
> > cpuset.mem_effective in demote_folio_list(), implement a new function
> > get_demotion_targets(), which returns a preferred demotion target
> > and all allowed (fallback) nodes against mems_effective,
> > and update demote_folio_list() and can_demote() accordingly to
> > use get_demotion_targets().
>
> 7d709f49babc fist appeared in 6.16, so we must decide whether to
> backport this fix into -stable kernels, via a Cc:
> <stable@vger.kernel.org>.
>
> To make this decision it's best to have a clear understanding of the
> userspace visible impact of the bug.  Putting pages into improper nodes
> is undesirable, but how much does it affect real-world workloads?
> Please include in the changelog some words about this to help others
> understand why we should backport the fix.
>
> > Furthermore, update some supporting functions:
> >   - Add a parameter for next_demotion_node() to return a copy of
> >     node_demotion[]->preferred, allowing get_demotion_targets()
> >     to select the next-best node for demotion.
> >   - Change the parameters for cpuset_node_allowed() and
> >     mem_cgroup_node_allowed() from nid to nodemask * to allow
> >     for direct logic-and operations with mems_effective.
>
> If we do decide to backport the fix into earlier kernels then it's best
> to keep the patch as small and as simple as possible.  So non-bugfix
> changes such as these are best made via a second followup patch which
> can be merged via the normal -rc staging process.
>
Hi Andrew, thank you for the review and suggestions.

I hvae sent a patch v2 for the backport. However, I forgot to add
the CC:stable line. I will fix it in v3.

Best,
Bing