vmscan,cgroup: apply mems_effective to reclaim

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>

1 files changed, 36 insertions, 0 deletions

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 54f6af362191a..83639a12883d1 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -4237,6 +4237,42 @@ bool cpuset_current_node_allowed(int node, gfp_t gfp_mask)
 	return allowed;
 }
 
+bool cpuset_node_allowed(struct cgroup *cgroup, int nid)
+{
+	struct cgroup_subsys_state *css;
+	struct cpuset *cs;
+	bool allowed;
+
+	/*
+	 * In v1, mem_cgroup and cpuset are unlikely in the same hierarchy
+	 * and mems_allowed is likely to be empty even if we could get to it,
+	 * so return true to avoid taking a global lock on the empty check.
+	 */
+	if (!cpuset_v2())
+		return true;
+
+	css = cgroup_get_e_css(cgroup, &cpuset_cgrp_subsys);
+	if (!css)
+		return true;
+
+	/*
+	 * Normally, accessing effective_mems would require the cpuset_mutex
+	 * or callback_lock - but node_isset is atomic and the reference
+	 * taken via cgroup_get_e_css is sufficient to protect css.
+	 *
+	 * Since this interface is intended for use by migration paths, we
+	 * relax locking here to avoid taking global locks - while accepting
+	 * there may be rare scenarios where the result may be innaccurate.
+	 *
+	 * Reclaim and migration are subject to these same race conditions, and
+	 * cannot make strong isolation guarantees, so this is acceptable.
+	 */
+	cs = container_of(css, struct cpuset, css);
+	allowed = node_isset(nid, cs->effective_mems);
+	css_put(css);
+	return allowed;
+}
+
 /**
  * cpuset_spread_node() - On which node to begin search for a page
  * @rotor: round robin rotor