mm/vmscan: fix demotion targets checks in reclaim/demotion

Patch series "mm/vmscan: fix demotion targets checks in reclaim/demotion",
v9.

This patch series addresses two issues in demote_folio_list(),
can_demote(), and next_demotion_node() in reclaim/demotion.

1. demote_folio_list() and can_demote() do not correctly check
   demotion target against cpuset.mems_effective, which will cause (a)
   pages to be demoted to not-allowed nodes and (b) pages fail demotion
   even if the system still has allowed demotion nodes.

   Patch 1 fixes this bug by updating cpuset_node_allowed() and
   mem_cgroup_node_allowed() to return effective_mems, allowing directly
   logic-and operation against demotion targets.

2. next_demotion_node() returns a preferred demotion target, but it
   does not check the node against allowed nodes.

   Patch 2 ensures that next_demotion_node() filters against the allowed
   node mask and selects the closest demotion target to the source node.


This patch (of 2):

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

Link: https://lkml.kernel.org/r/20260114205305.2869796-1-bingjiao@google.com
Link: https://lkml.kernel.org/r/20260114205305.2869796-2-bingjiao@google.com
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>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Gregory Price <gourry@gourry.net>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Waiman Long <longman@redhat.com>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 36 insertions, 18 deletions

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 99cf37e7d491..a5fc8d904dfa 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -4423,40 +4423,58 @@ bool cpuset_current_node_allowed(int node, gfp_t gfp_mask)
 	return allowed;
 }
 
-bool cpuset_node_allowed(struct cgroup *cgroup, int nid)
+/**
+ * cpuset_nodes_allowed - return effective_mems mask from a cgroup cpuset.
+ * @cgroup: pointer to struct cgroup.
+ * @mask: pointer to struct nodemask_t to be returned.
+ *
+ * Returns effective_mems mask from a cgroup cpuset if it is cgroup v2 and
+ * has cpuset subsys. Otherwise, returns node_states[N_MEMORY].
+ *
+ * This function intentionally avoids taking the cpuset_mutex or callback_lock
+ * when accessing effective_mems. This is because the obtained effective_mems
+ * is stale immediately after the query anyway (e.g., effective_mems is updated
+ * immediately after releasing the lock but before returning).
+ *
+ * As a result, returned @mask may be empty because cs->effective_mems can be
+ * rebound during this call. Besides, nodes in @mask are not guaranteed to be
+ * online due to hot plugins. Callers should check the mask for validity on
+ * return based on its subsequent use.
+ **/
+void cpuset_nodes_allowed(struct cgroup *cgroup, nodemask_t *mask)
 {
 	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.
+	 * so return directly to avoid taking a global lock on the empty check.
 	 */
-	if (!cpuset_v2())
-		return true;
+	if (!cgroup || !cpuset_v2()) {
+		nodes_copy(*mask, node_states[N_MEMORY]);
+		return;
+	}
 
 	css = cgroup_get_e_css(cgroup, &cpuset_cgrp_subsys);
-	if (!css)
-		return true;
+	if (!css) {
+		nodes_copy(*mask, node_states[N_MEMORY]);
+		return;
+	}
 
 	/*
-	 * 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.
+	 * The reference taken via cgroup_get_e_css is sufficient to
+	 * protect css, but it does not imply safe accesses to effective_mems.
 	 *
-	 * Reclaim and migration are subject to these same race conditions, and
-	 * cannot make strong isolation guarantees, so this is acceptable.
+	 * Normally, accessing effective_mems would require the cpuset_mutex
+	 * or callback_lock - but the correctness of this information is stale
+	 * immediately after the query anyway. We do not acquire the lock
+	 * during this process to save lock contention in exchange for racing
+	 * against mems_allowed rebinds.
 	 */
 	cs = container_of(css, struct cpuset, css);
-	allowed = node_isset(nid, cs->effective_mems);
+	nodes_copy(*mask, cs->effective_mems);
 	css_put(css);
-	return allowed;
 }
 
 /**