linux-kernel - [PATCH v1 2/2] mm/vmscan: select the closest perferred node in demote_folio

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Message-ID: <20260107072814.2324646-3-bingjiao@google.com>
Date: Wed,  7 Jan 2026 07:28:14 +0000
From: Bing Jiao <bingjiao@...gle.com>
To: linux-mm@...ck.org
Cc: Bing Jiao <bingjiao@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Johannes Weiner <hannes@...xchg.org>, David Hildenbrand <david@...nel.org>, Michal Hocko <mhocko@...nel.org>, 
	Qi Zheng <zhengqi.arch@...edance.com>, Shakeel Butt <shakeel.butt@...ux.dev>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Axel Rasmussen <axelrasmussen@...gle.com>, 
	Yuanchu Xie <yuanchu@...gle.com>, Wei Xu <weixugc@...gle.com>, linux-kernel@...r.kernel.org
Subject: [PATCH v1 2/2] mm/vmscan: select the closest perferred node in demote_folio_list()

The preferred demotion node should be the one closest to the source
node to minimize migration latency. However, if the preferred node
is not set in mems_allowed, demote_folio_list() currently randomly
select one from allowed nodes as the new preferred node. This can
result in selecting a very distant node.

Update demote_folio_list() to traverse the demotion targets
hierarchically until the perferred node is set in mems_allowed,
ensuring the perferred target is always the closest available node.

Signed-off-by: Bing Jiao <bingjiao@...gle.com>
---
 mm/vmscan.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index db2413c4bd26..d452974c946e 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1052,8 +1052,18 @@ static unsigned int demote_folio_list(struct list_head *demote_folios,
 	if (nodes_empty(allowed_mask))
 		return false;

-	if (!node_isset(target_nid, allowed_mask))
-		target_nid = node_random(&allowed_mask);
+	while (target_nid != NUMA_NO_NODE &&
+	       !node_isset(target_nid, allowed_mask)) {
+		/* Get the preferred demotion target from the next tier. */
+		target_nid = next_demotion_node(target_nid);
+	}
+
+	/*
+	 * The perferred node query is subject to race conditions such as
+	 * nodes in the next tier are hot-unplugged.
+	 */
+	if (target_nid == NUMA_NO_NODE)
+		return 0;
 	mtc.nid = target_nid;

 	/* Demotion ignores all cpuset and mempolicy settings */
--
2.52.0.358.g0dd7633a29-goog