mm/vmstat: fold stranded per-cpu node stats when a node comes online

A per-node vmstat counter is pgdat->vm_stat[] plus per-cpu deltas.  A
balanced counter can sit split as global=+N / per-cpu=-N.

The folds reconciling the split only walk online nodes, so when
try_offline_node() marks a node offline the per-cpu deltas are stranded.

A subsequent online resets the per-cpu area but not pgdat->vm_stat[],
orphaning the +N permanently.  All NR_VM_NODE_STAT_ITEMS are affected.

The existing code zeroes the per-cpu counters and causes a permanent skew.
Fold the stranded deltas instead, before the node rejoins the online set.
The node is not online yet and the hotplug lock is held, so the remote
access to per-cpu values is safe.

Discovered when node compaction hung for a nearly empty node, as the math
to determine throttling broke.  Reproduced by repeated memory
hotplug/unplug cycles on a node under pressure: NR_ISOLATED_ANON ratchets
up and never returns to zero.

Link: https://lore.kernel.org/20260627202243.758289-1-gourry@gourry.net
Fixes: 75ef71840539 ("mm, vmstat: add infrastructure for per-node vmstats")
Signed-off-by: Gregory Price <gourry@gourry.net>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 11 insertions, 4 deletions

diff --git a/mm/mm_init.c b/mm/mm_init.c
index 0f64909e8d20..498d62c4ece3 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -1540,7 +1540,7 @@ void __ref free_area_init_core_hotplug(struct pglist_data *pgdat)
 {
 	int nid = pgdat->node_id;
 	enum zone_type z;
-	int cpu;
+	int cpu, i;
 
 	pgdat_init_internals(pgdat);
 
@@ -1558,10 +1558,17 @@ void __ref free_area_init_core_hotplug(struct pglist_data *pgdat)
 	pgdat->node_start_pfn = 0;
 	pgdat->node_present_pages = 0;
 
-	for_each_online_cpu(cpu) {
-		struct per_cpu_nodestat *p;
+	/*
+	 * Hot-unplug can leave per-cpu vmstat deltas unfolded (folders skip
+	 * offline nodes) - reconcile this at online. Foreign access to counters
+	 * is safe: the node is not online yet and we hold the hotplug lock.
+	 */
+	for_each_possible_cpu(cpu) {
+		struct per_cpu_nodestat *p = per_cpu_ptr(pgdat->per_cpu_nodestats, cpu);
 
-		p = per_cpu_ptr(pgdat->per_cpu_nodestats, cpu);
+		for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++)
+			if (p->vm_node_stat_diff[i])
+				node_page_state_add(p->vm_node_stat_diff[i], pgdat, i);
 		memset(p, 0, sizeof(*p));
 	}