mm/vmscan: select the closest preferred node in demote_folio_list()

The preferred demotion node (migration_target_control.nid) should be the
one closest to the source node to minimize migration latency.  Currently,
a discrepancy exists where demote_folio_list() randomly selects an allowed
node if the preferred node from next_demotion_node() is not set in
mems_effective.

To address it, update next_demotion_node() to select a preferred target
against allowed nodes; and to return the closest demotion target if all
preferred nodes are not in mems_effective via next_demotion_node().

It ensures that the preferred demotion target is consistently the closest
available node to the source node.

[akpm@linux-foundation.org: fix comment typo, per Shakeel]
Link: https://lkml.kernel.org/r/20260114205305.2869796-3-bingjiao@google.com
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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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, const nodemask_t *allowed_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, const nodemask_t *allowed_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, const nodemask_t *allowed_mask)
 {
 	return NUMA_NO_NODE;
 }

mm/memory-tiers.c

@@ -320,16 +320,17 @@ 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
+ * @allowed_mask: The pointer to allowed node mask
  *
  * 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, const nodemask_t *allowed_mask)
 {
 	struct demotion_nodes *nd;
-	int target;
+	nodemask_t mask;
 
 	if (!node_demotion)
 		return NUMA_NO_NODE;
@@ -344,6 +345,10 @@ int next_demotion_node(int node)
 	 * node_demotion[] reads need to be consistent.
 	 */
 	rcu_read_lock();
+	/* Filter out nodes that are not in allowed_mask. */
+	nodes_and(mask, nd->preferred, *allowed_mask);
+	rcu_read_unlock();
+
 	/*
 	 * If there are multiple target nodes, just select one
 	 * target node randomly.
@@ -356,10 +361,16 @@ int next_demotion_node(int node)
 	 * caching issue, which seems more complicated. So selecting
 	 * target node randomly seems better until now.
 	 */
-	target = node_random(&nd->preferred);
-	rcu_read_unlock();
+	if (!nodes_empty(mask))
+		return node_random(&mask);
 
-	return target;
+	/*
+	 * Preferred nodes are not in allowed_mask. Flip bits in
+	 * allowed_mask as used node mask. Then, use it to get the
+	 * closest demotion target.
+	 */
+	nodes_complement(mask, *allowed_mask);
+	return find_next_best_node(node, &mask);
 }
 
 static void disable_all_demotion_targets(void)

mm/vmscan.c

@@ -1046,12 +1046,11 @@ static unsigned int demote_folio_list(struct list_head *demote_folios,
 	if (nodes_empty(allowed_mask))
 		return 0;
 
-	target_nid = next_demotion_node(pgdat->node_id);
+	target_nid = next_demotion_node(pgdat->node_id, &allowed_mask);
 	if (target_nid == NUMA_NO_NODE)
 		/* No lower-tier nodes or nodes were hot-unplugged. */
 		return 0;
-	if (!node_isset(target_nid, allowed_mask))
-		target_nid = node_random(&allowed_mask);
+
 	mtc.nid = target_nid;
 
 	/* Demotion ignores all cpuset and mempolicy settings */