[PATCH v2] mm/memory-tiers: reset demotion targets of nodes without memory

[Gregory Price (Meta)]

When a node loses its last memory, offline_pages() clears N_MEMORY
before the memory-tier callback rebuilds the demotion targets.
disable_all_demotion_targets() resets only N_MEMORY nodes, so the node
keeps its old node_demotion[] entry while it has no memory. Before
6c542ab75714, the reset covered every online node.

A reclaimer reads the stale entry only between the node regaining memory
and its targets being rebuilt, and it filters the entry against the
current allowed targets, so there is no user-visible effect. To see it,
online and then offline a node's only DIMM: a debugger still finds the
lower-tier node in node_demotion[nid].preferred.

Reset the targets of every node.

Fixes: 6c542ab75714 ("mm/demotion: build demotion targets based on explicit memory tiers")
Signed-off-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
---
v2: +comment

 mm/memory-tiers.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
index 25e121851b58..9d872404592e 100644
--- a/mm/memory-tiers.c
+++ b/mm/memory-tiers.c
@@ -378,7 +378,11 @@ static void disable_all_demotion_targets(void)
 	struct memory_tier *memtier;
 	int node;
 
-	for_each_node_state(node, N_MEMORY) {
+	/*
+	 * When the NODE_REMOVED_LAST_MEMORY event fires, N_MEMORY has already
+	 * been dropped from the node, so we must walk every node instead.
+	 */
+	for_each_node(node) {
 		node_demotion[node].preferred = NODE_MASK_NONE;
 		/*
 		 * We are holding memory_tier_lock, it is safe
-- 
2.56.0