Re: [PATCH v2] mm/vmstat: fold stranded per-cpu node stats when a node comes online

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 27 Jun 2026 16:22:43 -0400 Gregory Price <gourry@gourry.net> wrote:

> 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.

Geeze, simple mistake, been there ten years...

> 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.

Oh.  Shouldn't we be doing this during offlining?

> 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.
> 
> ...
>
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -1536,7 +1536,7 @@ 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);
>  
> @@ -1554,10 +1554,17 @@ 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) {

That's a lot of CPUs

> +		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++)

and that's a lot of items.

I guess the overall loop count won't be large enough to cause issues,
but it's large!

Perhaps there's some simple test we can do on the per_cpu_nodestat to
avoid the inner loop?  Perhaps might need to add a field for this?

btw, "for(int i..." is allowed nowadays.  It'll make this code nicer, IMO.

And... Sashiko seems to have found a pre-existing issue:
	https://sashiko.dev/#/patchset/20260627202243.758289-1-gourry@gourry.net

> +			if (p->vm_node_stat_diff[i])
> +				node_page_state_add(p->vm_node_stat_diff[i], pgdat, i);
>  		memset(p, 0, sizeof(*p));
>  	}