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It is possible for a reclaimer to cause demotions of an lruvec belonging to a cgroup with cpuset.mems set to exclude some nodes. Attempt to apply this limitation based on the lruvec's memcg and prevent demotion. Notably, this may still allow demotion of shared libraries or any memory first instantiated in another cgroup. This means cpusets still cannot cannot guarantee complete isolation when demotion is enabled, and the docs have been updated to reflect this. This is useful for isolating workloads on a multi-tenant system from certain classes of memory more consistently - with the noted exceptions. Note on locking: The cgroup_get_e_css reference protects the css->effective_mems, and calls of this interface would be subject to the same race conditions associated with a non-atomic access to cs->effective_mems. So while this interface cannot make strong guarantees of correctness, it can therefore avoid taking a global or rcu_read_lock for performance. Link: https://lkml.kernel.org/r/20250424202806.52632-3-gourry@gourry.net Signed-off-by: Gregory Price <gourry@gourry.net> Suggested-by: Shakeel Butt <shakeel.butt@linux.dev> Suggested-by: Waiman Long <longman@redhat.com> Acked-by: Tejun Heo <tj@kernel.org> Acked-by: Johannes Weiner <hannes@cmpxchg.org> Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev> Reviewed-by: Waiman Long <longman@redhat.com> Cc: Michal Hocko <mhocko@kernel.org> Cc: Michal Koutný <mkoutny@suse.com> Cc: Muchun Song <muchun.song@linux.dev> Cc: Roman Gushchin <roman.gushchin@linux.dev> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>