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

[Michal Hocko]

On Wed 03-12-25 10:15:04, David Hildenbrand (Red Hat) wrote:
> 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.

Right. But what should the hotplug code do under that condition. Loop
for ever or fail reporting the broken contract? I would lean towards the
latter. We have never promised that offlining will not fail ever for
movable zones. We just guarantee that the operation is resistant against
recovarable failures.

-- 
Michal Hocko
SUSE Labs