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

[Gregory Price <gourry@gourry.net>]

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.

~Gregory