Re: [PATCH] mm/page_alloc: make percpu_pagelist_high_fraction reads lock-free

[David Hildenbrand (Red Hat)]

On 12/3/25 09:59, Gregory Price wrote:
> On Wed, Dec 03, 2025 at 09:42:59AM +0100, Michal Hocko wrote:
> > On Wed 03-12-25 03:35:51, Gregory Price wrote:
> > > 		if (!ret) {
> > > 			/*
> > > 			 * TODO: fatal migration failures should bail
> > > 			 * out
> > > 			 */
> > > 			do_migrate_range(pfn, end_pfn);
> > > 		}
> > >
> > > Maybe it's time to implement the bail out?
> >
> > That would be great but can we tell transient from permanent migration
> > failures? Maybe long term pins could be treated as permanent failure.
>
> I see deep in migration code `migrate_pages_batch()` we would return
> "Some other failure" as fatal:
>
> 	switch(rc) {
> 	case -ENOMEM:
> 		...
> 		/* Note: some long-term pin handing is done here */
> 		break;
> 	case -EAGAIN:
> 		...
> 		break;
> 	case 0:
> 		...
> 		list_move_tail(&folio->lru, &unmap_folios);
> 		list_add_tail(&dst->lru, &dst_folios);
> 		break;
> 	default:
> 		/*
> 		 * Permanent failure (-EBUSY, etc.):
> 		 * unlike -EAGAIN case, the failed folio is
> 		 * removed from migration folio list and not
> 		 * retried in the next outer loop.
> 		 */
> 		nr_failed++;
> 		stats->nr_thp_failed += is_thp;
> 		stats->nr_failed_pages += nr_pages;
> 		break;
> 	}
>
> So at a minimum we could at least check for !(ENOMEM,EAGAIN) I suppose?
>
> It's unclear to me based on this code here how longerm pinning would
> return.  Maybe David knows.

I would assume that additional references will always result in -EAGAIN. 
Remember that we cannot distinguish short-term pins from long-term pins.

We should never have longterm-pins on ZONE_MOVABLE, unless something 
broke that contract and needs to be fixed.

--
Cheers

David