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

[Gregory Price <gourry@gourry.net>]

On Sat, Jun 27, 2026 at 04:10:07PM -0700, Andrew Morton wrote:
> On Sat, 27 Jun 2026 16:22:43 -0400 Gregory Price <gourry@gourry.net> wrote:
> 
> > +		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?
>

I took a look, but that would involve adding another per-cpu field and
then making sure all the races on that field are respected as well.

Not sure it's worth it for such an extremely rare event.

I can try to get clever on the folding logic if you'd like, let me know.

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

Otherwise i can send you a respin for this.

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

Incoming patch for this shortly.  Pretty trivial.

~Gregory