Message-ID: <20251220061022.2726028-1-bingjiao@google.com>
Date: Sat, 20 Dec 2025 06:10:21 +0000
From: Bing Jiao <bingjiao@...gle.com>
To: linux-mm@...ck.org
Cc: gourry@...rry.net, Bing Jiao <bingjiao@...gle.com>, 
	Waiman Long <longman@...hat.com>, Johannes Weiner <hannes@...xchg.org>, 
	Michal Hocko <mhocko@...nel.org>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Shakeel Butt <shakeel.butt@...ux.dev>, Muchun Song <muchun.song@...ux.dev>, 
	Andrew Morton <akpm@...ux-foundation.org>, David Hildenbrand <david@...nel.org>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>, 
	Tejun Heo <tj@...nel.org>, "Michal Koutný" <mkoutny@...e.com>, Qi Zheng <zhengqi.arch@...edance.com>, 
	Axel Rasmussen <axelrasmussen@...gle.com>, Yuanchu Xie <yuanchu@...gle.com>, 
	Wei Xu <weixugc@...gle.com>, cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] mm/vmscan: respect mems_effective in demote_folio_list()

Commit 7d709f49babc ("vmscan,cgroup: apply mems_effective to reclaim")
introduces the cpuset.mems_effective check and applies it to
can_demote(). However, 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.

To address the issue that demotion targets do not respect
cpuset.mem_effective in demote_folio_list(), implement a new function
get_demotion_targets(), which returns a preferred demotion target
and all allowed (fallback) nodes against mems_effective,
and update demote_folio_list() and can_demote() accordingly to
use get_demotion_targets().

Furthermore, update some supporting functions:
  - Add a parameter for next_demotion_node() to return a copy of
    node_demotion[]->preferred, allowing get_demotion_targets()
    to select the next-best node for demotion.
  - Change the parameters for cpuset_node_allowed() and
    mem_cgroup_node_allowed() from nid to nodemask * to allow
    for direct logic-and operations with mems_effective.

Signed-off-by: Bing Jiao <bingjiao@...gle.com>
---
 include/linux/cpuset.h       |  5 +--
 include/linux/memcontrol.h   |  6 +--
 include/linux/memory-tiers.h |  6 +--
 kernel/cgroup/cpuset.c       | 14 +++----
 mm/memcontrol.c              |  5 ++-
 mm/memory-tiers.c            |  8 +++-
 mm/vmscan.c                  | 77 +++++++++++++++++++++++++++++-------
 7 files changed, 87 insertions(+), 34 deletions(-)

diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
index a98d3330385c..27a0b6e9fb9d 100644
--- a/include/linux/cpuset.h
+++ b/include/linux/cpuset.h
@@ -174,7 +174,7 @@ 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_node_allowed(struct cgroup *cgroup, nodemask_t *nodes);
 #else /* !CONFIG_CPUSETS */

 static inline bool cpusets_enabled(void) { return false; }
@@ -301,9 +301,8 @@ 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_node_allowed(struct cgroup *cgroup, nodemask_t *nodes)
 {
-	return true;
 }
 #endif /* !CONFIG_CPUSETS */

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index fd400082313a..a87f008b6600 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_allowed(struct mem_cgroup *memcg, nodemask_t *nodes);

 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_allowed(struct mem_cgroup *memcg,
+					   nodemask_t *nodes)
 {
-	return true;
 }

 static inline void mem_cgroup_show_protected_memory(struct mem_cgroup *memcg)
diff --git a/include/linux/memory-tiers.h b/include/linux/memory-tiers.h
index 7a805796fcfd..2706ebfa94b5 100644
--- a/include/linux/memory-tiers.h
+++ b/include/linux/memory-tiers.h
@@ -53,11 +53,11 @@ struct memory_dev_type *mt_find_alloc_memory_type(int adist,
 						  struct list_head *memory_types);
 void mt_put_memory_types(struct list_head *memory_types);
 #ifdef CONFIG_MIGRATION
-int next_demotion_node(int node);
+int next_demotion_node(int node, nodemask_t *mask);
 void node_get_allowed_targets(pg_data_t *pgdat, nodemask_t *targets);
 bool node_is_toptier(int node);
 #else
-static inline int next_demotion_node(int node)
+static inline int next_demotion_node(int node, nodemask_t *mask)
 {
 	return NUMA_NO_NODE;
 }
@@ -101,7 +101,7 @@ static inline void clear_node_memory_type(int node, struct memory_dev_type *memt

 }

-static inline int next_demotion_node(int node)
+static inline int next_demotion_node(int node, nodemask_t *mask)
 {
 	return NUMA_NO_NODE;
 }
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 6e6eb09b8db6..2d78cfde5911 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -4416,11 +4416,10 @@ bool cpuset_current_node_allowed(int node, gfp_t gfp_mask)
 	return allowed;
 }

-bool cpuset_node_allowed(struct cgroup *cgroup, int nid)
+void cpuset_node_allowed(struct cgroup *cgroup, nodemask_t *nodes)
 {
 	struct cgroup_subsys_state *css;
 	struct cpuset *cs;
-	bool allowed;

 	/*
 	 * In v1, mem_cgroup and cpuset are unlikely in the same hierarchy
@@ -4428,16 +4427,16 @@ bool cpuset_node_allowed(struct cgroup *cgroup, int nid)
 	 * so return true to avoid taking a global lock on the empty check.
 	 */
 	if (!cpuset_v2())
-		return true;
+		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
-	 * taken via cgroup_get_e_css is sufficient to protect css.
+	 * or callback_lock - but 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
@@ -4447,9 +4446,8 @@ 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(*nodes, *nodes, cs->effective_mems);
 	css_put(css);
-	return allowed;
 }

 /**
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 75fc22a33b28..a62c75b136ef 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_allowed(struct mem_cgroup *memcg, nodemask_t *nodes)
 {
-	return memcg ? cpuset_node_allowed(memcg->css.cgroup, nid) : true;
+	if (memcg)
+		cpuset_node_allowed(memcg->css.cgroup, nodes);
 }

 void mem_cgroup_show_protected_memory(struct mem_cgroup *memcg)
diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
index 20aab9c19c5e..ed0ee9c3ae70 100644
--- a/mm/memory-tiers.c
+++ b/mm/memory-tiers.c
@@ -320,13 +320,14 @@ void node_get_allowed_targets(pg_data_t *pgdat, nodemask_t *targets)
 /**
  * next_demotion_node() - Get the next node in the demotion path
  * @node: The starting node to lookup the next node
+ * @mask: The preferred nodemask copy to be returned
  *
  * Return: node id for next memory node in the demotion path hierarchy
  * from @node; NUMA_NO_NODE if @node is terminal.  This does not keep
  * @node online or guarantee that it *continues* to be the next demotion
  * target.
  */
-int next_demotion_node(int node)
+int next_demotion_node(int node, nodemask_t *mask)
 {
 	struct demotion_nodes *nd;
 	int target;
@@ -355,7 +356,12 @@ int next_demotion_node(int node)
 	 * last target node. Or introducing per-cpu data to avoid
 	 * caching issue, which seems more complicated. So selecting
 	 * target node randomly seems better until now.
+	 *
+	 * Copy preferred nodes as the fallback if the returned one
+	 * does not satisify some constraints like cpuset.
 	 */
+	if (mask)
+		nodes_copy(*mask, nd->preferred);
 	target = node_random(&nd->preferred);
 	rcu_read_unlock();

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 8bdb1629b6eb..2ddbf5584af8 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -341,22 +341,71 @@ static void flush_reclaim_state(struct scan_control *sc)
 	}
 }

+/*
+ * Returns a preferred demotion node and all allowed demotion @targets.
+ * Returns NUMA_NO_NODE and @targets is meaningless if no allowed nodes.
+ */
+static int get_demotion_targets(nodemask_t *targets, struct pglist_data *pgdat,
+				struct mem_cgroup *memcg)
+{
+	nodemask_t allowed_mask;
+	nodemask_t preferred_mask;
+	int preferred_node;
+
+	if (!pgdat)
+		return NUMA_NO_NODE;
+
+	preferred_node = next_demotion_node(pgdat->node_id, &preferred_mask);
+	if (preferred_node == NUMA_NO_NODE)
+		return NUMA_NO_NODE;
+
+	node_get_allowed_targets(pgdat, &allowed_mask);
+	mem_cgroup_node_allowed(memcg, &allowed_mask);
+	if (nodes_empty(allowed_mask))
+		return NUMA_NO_NODE;
+
+	if (targets)
+		nodes_copy(*targets, allowed_mask);
+
+	do {
+		if (node_isset(preferred_node, allowed_mask))
+			return preferred_node;
+
+		nodes_and(preferred_mask, preferred_mask, allowed_mask);
+		if (!nodes_empty(preferred_mask))
+			return node_random(&preferred_mask);
+
+		/*
+		 * Hop to the next tier of preferred nodes. Even if
+		 * preferred_node is not set in allowed_mask, still can use it
+		 * to query the nest-best demotion nodes.
+		 */
+		preferred_node = next_demotion_node(preferred_node,
+						    &preferred_mask);
+	} while (preferred_node != NUMA_NO_NODE);
+
+	/*
+	 * Should not reach here, as a non-empty allowed_mask ensures
+	 * there must have a target node for demotion.
+	 * Otherwise, it suggests something wrong in node_demotion[]->preferred,
+	 * where the same-tier nodes have different preferred targets.
+	 * E.g., if node 0 identifies both nodes 2 and 3 as preferred targets,
+	 * but nodes 2 and 3 themselves have different preferred nodes.
+	 */
+	WARN_ON_ONCE(1);
+	return node_random(&allowed_mask);
+}
+
 static bool can_demote(int nid, struct scan_control *sc,
 		       struct mem_cgroup *memcg)
 {
-	int demotion_nid;
-
 	if (!numa_demotion_enabled)
 		return false;
 	if (sc && sc->no_demotion)
 		return false;

-	demotion_nid = next_demotion_node(nid);
-	if (demotion_nid == NUMA_NO_NODE)
-		return false;
-
-	/* If demotion node isn't in the cgroup's mems_allowed, fall back */
-	return mem_cgroup_node_allowed(memcg, demotion_nid);
+	return get_demotion_targets(NULL, NODE_DATA(nid), memcg) !=
+	       NUMA_NO_NODE;
 }

 static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,
@@ -1019,9 +1068,10 @@ 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);
+	int target_nid;
 	unsigned int nr_succeeded;
 	nodemask_t allowed_mask;

@@ -1033,7 +1083,6 @@ 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,
 	};
@@ -1041,10 +1090,10 @@ static unsigned int demote_folio_list(struct list_head *demote_folios,
 	if (list_empty(demote_folios))
 		return 0;

+	target_nid = get_demotion_targets(&allowed_mask, pgdat, memcg);
 	if (target_nid == NUMA_NO_NODE)
 		return 0;
-
-	node_get_allowed_targets(pgdat, &allowed_mask);
+	mtc.nid = target_nid;

 	/* Demotion ignores all cpuset and mempolicy settings */
 	migrate_pages(demote_folios, alloc_demote_folio, NULL,
@@ -1566,7 +1615,7 @@ 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 */
--
2.52.0.351.gbe84eed79e-goog

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