FAILED: patch "[PATCH] mm/vmstat: fold stranded per-cpu node stats when a node comes" failed to apply to 6.1-stable tree

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x ea3034b2b00fa50c8d2518d0804c9d427bbafa86
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026080556-undermost-commerce-4a9a@gregkh' --subject-prefix 'PATCH 6.1.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From ea3034b2b00fa50c8d2518d0804c9d427bbafa86 Mon Sep 17 00:00:00 2001
From: Gregory Price <[email protected]>
Date: Sat, 27 Jun 2026 16:22:43 -0400
Subject: [PATCH] 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.

index 0f64909e8d20..498d62c4ece3 100644
@@ -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));
 	}
 

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.

---
 mm/page_alloc.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

index 93c63b5fad288..6ce12838f0fad 100644
@@ -7870,7 +7870,7 @@ 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);
 

@@ -7886,10 +7886,18 @@ void __ref free_area_init_core_hotplug(struct pglist_data *pgdat)
 	pgdat->kswapd_order = 0;
 	pgdat->kswapd_highest_zoneidx = 0;
 	pgdat->node_start_pfn = 0;
-	for_each_online_cpu(cpu) {
-		struct per_cpu_nodestat *p;
 
-		p = per_cpu_ptr(pgdat->per_cpu_nodestats, cpu);
+	/*
+	 * 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);
+
+		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