From 421e447b53c70f3a897a5d9da775eda1da84fb22 Mon Sep 17 00:00:00 2001 From: Gregory Price Date: Sat, 27 Jun 2026 16:22:43 -0400 Subject: [PATCH] mm/vmstat: fold stranded per-cpu node stats when a node comes online commit ea3034b2b00fa50c8d2518d0804c9d427bbafa86 upstream. 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 Cc: Johannes Weiner Cc: Mel Gorman Cc: Mike Rapoport Cc: Vlastimil Babka Cc: Signed-off-by: Andrew Morton Signed-off-by: Greg Kroah-Hartman --- mm/mm_init.c | 15 +++++++++++---- 1 file changed, 11 insertions(+), 4 deletions(-) diff --git a/mm/mm_init.c b/mm/mm_init.c index a71fbb3cec38..1aba3e8737d8 100644 --- a/mm/mm_init.c +++ b/mm/mm_init.c @@ -1514,7 +1514,7 @@ 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); @@ -1532,10 +1532,17 @@ 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)); }