linux-kernel - Re: [PATCH v3] mm/vmscan: fix demotion targets checks in reclaim/demotion

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Message-ID: <24a480ec-c7b3-4e0d-9895-f9c2ad88dc47@redhat.com>
Date: Fri, 26 Dec 2025 14:32:23 -0500
From: Waiman Long <llong@...hat.com>
To: Bing Jiao <bingjiao@...gle.com>, linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
 gourry@...rry.net, hannes@...xchg.org, mhocko@...nel.org,
 roman.gushchin@...ux.dev, shakeel.butt@...ux.dev, muchun.song@...ux.dev,
 tj@...nel.org, mkoutny@...e.com, david@...nel.org,
 zhengqi.arch@...edance.com, lorenzo.stoakes@...cle.com,
 axelrasmussen@...gle.com, chenridong@...weicloud.com, yuanchu@...gle.com,
 weixugc@...gle.com, cgroups@...r.kernel.org
Subject: Re: [PATCH v3] mm/vmscan: fix demotion targets checks in
 reclaim/demotion


On 12/23/25 4:19 PM, Bing Jiao wrote:
> Fix two bugs in demote_folio_list() and can_demote() due to incorrect
> demotion target checks in reclaim/demotion.
>
> Commit 7d709f49babc ("vmscan,cgroup: apply mems_effective to reclaim")
> introduces the cpuset.mems_effective check and applies it to
> can_demote(). However:
>
>    1. 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.
>
>    2. It checks only the nodes in the immediate next demotion hierarchy
>       and does not check all allowed demotion targets in can_demote().
>       This can cause pages to never be demoted if the nodes in the next
>       demotion hierarchy are not set in mems_effective.
>
> These bugs break resource isolation provided by cpuset.mems.
> This is visible from userspace because pages can either fail to be
> demoted entirely or are demoted to nodes that are not allowed
> in multi-tier memory systems.
>
> To address these bugs, update cpuset_node_allowed() and
> mem_cgroup_node_allowed() to return effective_mems, allowing directly
> logic-and operation against demotion targets. Also update can_demote()
> and demote_folio_list() accordingly.
>
> Reproduct Bug 1:

"Reproduct" is not an English word. Use either "Reproduce" or 
"Reproduction".

Cheers,
Longman