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

["David Hildenbrand (Red Hat)" <david@kernel.org>]

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