[PATCH v4] mm/vmscan: fix demotion targets checks in reclaim/demotion

[Bing Jiao <bingjiao@google.com>]

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.

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")
Cc: <stable@vger.kernel.org>
Signed-off-by: Bing Jiao <bingjiao@google.com>

---
v3 -> v4:
Update functions to filter out nodes that are not in cgroup's
mems_allowed rather than returning mems_allowed directly.
It minimizes stack usage and remains versatile enough to return
mems_allowed when all possible nodes are set in the passed
nodemask_t pointer.
---

 include/linux/cpuset.h     |  7 ++++---
 include/linux/memcontrol.h |  6 +++---
 kernel/cgroup/cpuset.c     | 24 +++++++++++++++---------
 mm/memcontrol.c            |  5 +++--
 mm/vmscan.c                | 32 +++++++++++++++++++++-----------
 5 files changed, 46 insertions(+), 28 deletions(-)

diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
index a98d3330385c..c937537f318a 100644
--- a/include/linux/cpuset.h
+++ b/include/linux/cpuset.h
@@ -174,7 +174,8 @@ static inline void set_mems_allowed(nodemask_t nodemask)
 	task_unlock(current);
 }

-extern bool cpuset_node_allowed(struct cgroup *cgroup, int nid);
+extern void cpuset_nodes_filter_allowed(struct cgroup *cgroup,
+					nodemask_t *mask);
 #else /* !CONFIG_CPUSETS */

 static inline bool cpusets_enabled(void) { return false; }
@@ -301,9 +302,9 @@ static inline bool read_mems_allowed_retry(unsigned int seq)
 	return false;
 }

-static inline bool cpuset_node_allowed(struct cgroup *cgroup, int nid)
+static inline void cpuset_nodes_filter_allowed(struct cgroup *cgroup,
+					       nodemask_t *mask)
 {
-	return true;
 }
 #endif /* !CONFIG_CPUSETS */

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index fd400082313a..911e0c71453e 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1740,7 +1740,7 @@ static inline void count_objcg_events(struct obj_cgroup *objcg,
 	rcu_read_unlock();
 }

-bool mem_cgroup_node_allowed(struct mem_cgroup *memcg, int nid);
+void mem_cgroup_node_filter_allowed(struct mem_cgroup *memcg, nodemask_t *mask);

 void mem_cgroup_show_protected_memory(struct mem_cgroup *memcg);

@@ -1811,9 +1811,9 @@ static inline ino_t page_cgroup_ino(struct page *page)
 	return 0;
 }

-static inline bool mem_cgroup_node_allowed(struct mem_cgroup *memcg, int nid)
+static inline void mem_cgroup_node_filter_allowed(struct mem_cgroup *memcg,
+						  nodemask_t *mask)
 {
-	return true;
 }

 static inline void mem_cgroup_show_protected_memory(struct mem_cgroup *memcg)
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 6e6eb09b8db6..cc0e1d42611c 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -4416,27 +4416,34 @@ bool cpuset_current_node_allowed(int node, gfp_t gfp_mask)
 	return allowed;
 }

-bool cpuset_node_allowed(struct cgroup *cgroup, int nid)
+/**
+ * cpuset_nodes_filter_allowed - filter out nodes not in cgroup's mems_allowed.
+ * @cgroup: pointer to struct cgroup.
+ * @mask: pointer to struct nodemask_t to be filtered.
+ *
+ * Description: Filters out nodes that are not in mems_allowed for @cgroup.
+ * Nodes returned in @mask are not guaranteed to be online.
+ **/
+void cpuset_nodes_filter_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())
+		return;

 	css = cgroup_get_e_css(cgroup, &cpuset_cgrp_subsys);
 	if (!css)
-		return true;
+		return;

 	/*
 	 * Normally, accessing effective_mems would require the cpuset_mutex
-	 * or callback_lock - but node_isset is atomic and the reference
+	 * or callback_lock - but not doing so is acceptable 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
@@ -4447,9 +4454,8 @@ bool cpuset_node_allowed(struct cgroup *cgroup, int nid)
 	 * cannot make strong isolation guarantees, so this is acceptable.
 	 */
 	cs = container_of(css, struct cpuset, css);
-	allowed = node_isset(nid, cs->effective_mems);
+	nodes_and(*mask, *mask, cs->effective_mems);
 	css_put(css);
-	return allowed;
 }

 /**
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 75fc22a33b28..4c850805b7a9 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5597,9 +5597,10 @@ subsys_initcall(mem_cgroup_swap_init);

 #endif /* CONFIG_SWAP */

-bool mem_cgroup_node_allowed(struct mem_cgroup *memcg, int nid)
+void mem_cgroup_node_filter_allowed(struct mem_cgroup *memcg, nodemask_t *mask)
 {
-	return memcg ? cpuset_node_allowed(memcg->css.cgroup, nid) : true;
+	if (memcg)
+		cpuset_nodes_filter_allowed(memcg->css.cgroup, mask);
 }

 void mem_cgroup_show_protected_memory(struct mem_cgroup *memcg)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index ac16b6b984ab..919e116ddaf3 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -345,18 +345,24 @@ static bool can_demote(int nid, struct scan_control *sc,
 		       struct mem_cgroup *memcg)
 {
 	int demotion_nid;
+	struct pglist_data *pgdat = NODE_DATA(nid);
+	nodemask_t allowed_mask;

-	if (!numa_demotion_enabled)
+	if (!pgdat || !numa_demotion_enabled)
 		return false;
 	if (sc && sc->no_demotion)
 		return false;

-	demotion_nid = next_demotion_node(nid);
-	if (demotion_nid == NUMA_NO_NODE)
+	node_get_allowed_targets(pgdat, &allowed_mask);
+	if (nodes_empty(allowed_mask))
+		return false;
+
+	/* Filter out nodes that are not in cgroup's mems_allowed. */
+	mem_cgroup_node_filter_allowed(memcg, &allowed_mask);
+	if (nodes_empty(allowed_mask))
 		return false;

-	/* If demotion node isn't in the cgroup's mems_allowed, fall back */
-	if (mem_cgroup_node_allowed(memcg, demotion_nid)) {
+	for_each_node_mask(demotion_nid, allowed_mask) {
 		int z;
 		struct zone *zone;
 		struct pglist_data *pgdat = NODE_DATA(demotion_nid);
@@ -1029,7 +1035,8 @@ static struct folio *alloc_demote_folio(struct folio *src,
  * Folios which are not demoted are left on @demote_folios.
  */
 static unsigned int demote_folio_list(struct list_head *demote_folios,
-				     struct pglist_data *pgdat)
+				      struct pglist_data *pgdat,
+				      struct mem_cgroup *memcg)
 {
 	int target_nid = next_demotion_node(pgdat->node_id);
 	unsigned int nr_succeeded;
@@ -1043,7 +1050,6 @@ static unsigned int demote_folio_list(struct list_head *demote_folios,
 		 */
 		.gfp_mask = (GFP_HIGHUSER_MOVABLE & ~__GFP_RECLAIM) |
 			__GFP_NOMEMALLOC | GFP_NOWAIT,
-		.nid = target_nid,
 		.nmask = &allowed_mask,
 		.reason = MR_DEMOTION,
 	};
@@ -1051,10 +1057,14 @@ static unsigned int demote_folio_list(struct list_head *demote_folios,
 	if (list_empty(demote_folios))
 		return 0;

-	if (target_nid == NUMA_NO_NODE)
-		return 0;
-
 	node_get_allowed_targets(pgdat, &allowed_mask);
+	mem_cgroup_node_filter_allowed(memcg, &allowed_mask);
+	if (nodes_empty(allowed_mask))
+		return false;
+
+	if (!node_isset(target_nid, allowed_mask))
+		target_nid = node_random(&allowed_mask);
+	mtc.nid = target_nid;

 	/* Demotion ignores all cpuset and mempolicy settings */
 	migrate_pages(demote_folios, alloc_demote_folio, NULL,
@@ -1576,7 +1586,7 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 	/* 'folio_list' is always empty here */

 	/* Migrate folios selected for demotion */
-	nr_demoted = demote_folio_list(&demote_folios, pgdat);
+	nr_demoted = demote_folio_list(&demote_folios, pgdat, memcg);
 	nr_reclaimed += nr_demoted;
 	stat->nr_demoted += nr_demoted;
 	/* Folios that could not be demoted are still in @demote_folios */
--
2.52.0.358.g0dd7633a29-goog