linux-kernel - Re: [PATCH] mm/page_alloc: make percpu_pagelist_high

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Message-ID: <aS_8BgwR3R_geayy@gourry-fedora-PF4VCD3F>
Date: Wed, 3 Dec 2025 03:59:50 -0500
From: Gregory Price <gourry@...rry.net>
To: Michal Hocko <mhocko@...e.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Aboorva Devarajan <aboorvad@...ux.ibm.com>, vbabka@...e.cz,
	surenb@...gle.com, jackmanb@...gle.com, hannes@...xchg.org,
	ziy@...dia.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Oscar Salvador <OSalvador@...e.com>,
	David Hildenbrand <david@...nel.org>
Subject: Re: [PATCH] mm/page_alloc: make percpu_pagelist_high_fraction reads
 lock-free

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