[PATCH 5.15.y] mm/vmstat: fold stranded per-cpu node stats when a node comes online

From: Gregory Price <[email protected]>

[ Upstream commit ea3034b2b00fa50c8d2518d0804c9d427bbafa86 ]

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/[email protected]
Fixes: 75ef71840539 ("mm, vmstat: add infrastructure for per-node vmstats")
Signed-off-by: Gregory Price <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>
---
 mm/memory_hotplug.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 2d8e9fb4ce0b2..bf0bc71510f99 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1190,7 +1190,7 @@ static pg_data_t __ref *hotadd_new_pgdat(int nid)
 			alloc_percpu(struct per_cpu_nodestat);
 		arch_refresh_nodedata(nid, pgdat);
 	} else {
-		int cpu;
+		int cpu, i;
 		/*
 		 * Reset the nr_zones, order and highest_zoneidx before reuse.
 		 * Note that kswapd will init kswapd_highest_zoneidx properly
@@ -1199,10 +1199,17 @@ static pg_data_t __ref *hotadd_new_pgdat(int nid)
 		pgdat->nr_zones = 0;
 		pgdat->kswapd_order = 0;
 		pgdat->kswapd_highest_zoneidx = 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));
 		}
 	}
-- 
2.53.0