Re: [PATCH v9 1/2] mm/vmscan: fix demotion targets checks in reclaim/demotion

[Shakeel Butt <shakeel.butt@linux.dev>]

On Wed, Jan 14, 2026 at 08:53:02PM +0000, Bing Jiao wrote:
> Fix two bugs in demote_folio_list() and can_demote() due to incorrect
> demotion target checks against cpuset.mems_effective 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.
> 
> Bug 1 reproduction:
>   Assume a system with 4 nodes, where nodes 0-1 are top-tier and
>   nodes 2-3 are far-tier memory. All nodes have equal capacity.
> 
>   Test script:
>     echo 1 > /sys/kernel/mm/numa/demotion_enabled
>     mkdir /sys/fs/cgroup/test
>     echo +cpuset > /sys/fs/cgroup/cgroup.subtree_control
>     echo "0-2" > /sys/fs/cgroup/test/cpuset.mems
>     echo $$ > /sys/fs/cgroup/test/cgroup.procs
>     swapoff -a
>     # Expectation: Should respect node 0-2 limit.
>     # Observation: Node 3 shows significant allocation (MemFree drops)
>     stress-ng --oomable --vm 1 --vm-bytes 150% --mbind 0,1
> 
> Bug 2 reproduction:
>   Assume a system with 6 nodes, where nodes 0-2 are top-tier,
>   node 3 is a far-tier node, and nodes 4-5 are the farthest-tier nodes.
>   All nodes have equal capacity.
> 
>   Test script:
>     echo 1 > /sys/kernel/mm/numa/demotion_enabled
>     mkdir /sys/fs/cgroup/test
>     echo +cpuset > /sys/fs/cgroup/cgroup.subtree_control
>     echo "0-2,4-5" > /sys/fs/cgroup/test/cpuset.mems
>     echo $$ > /sys/fs/cgroup/test/cgroup.procs
>     swapoff -a
>     # Expectation: Pages are demoted to Nodes 4-5
>     # Observation: No pages are demoted before oom.
>     stress-ng --oomable --vm 1 --vm-bytes 150% --mbind 0,1,2
> 
> Fixes: 7d709f49babc ("vmscan,cgroup: apply mems_effective to reclaim")
> Signed-off-by: Bing Jiao <bingjiao@google.com>

Acked-by: Shakeel Butt <shakeel.butt@linux.dev>