linux-kernel - Re: [PATCH] mm/migrate: Putback split folios when numa hint migration fails

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Message-Id: <20240708160407.a0c51eb11d0403c161d27540@linux-foundation.org>
Date: Mon, 8 Jul 2024 16:04:07 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Peter Xu <peterx@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, Zi Yan
 <ziy@...dia.com>, Yang Shi <shy828301@...il.com>, Hugh Dickins
 <hughd@...gle.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>, Huang Ying
 <ying.huang@...el.com>, David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH] mm/migrate: Putback split folios when numa hint
 migration fails

On Mon,  8 Jul 2024 17:55:37 -0400 Peter Xu <peterx@...hat.com> wrote:

> This issue is not from any report yet, but by code observation only.
> 
> This is yet another fix besides Hugh's patch [1] but on relevant code path,
> where eager split of folio can happen if the folio is already on deferred
> list during a folio migration.
> 
> Here the issue is NUMA path (migrate_misplaced_folio()) may start to
> encounter such folio split now even with MR_NUMA_MISPLACED hint applied.
> Then when migrate_pages() didn't migrate all the folios, it's possible the
> split small folios be put onto the list instead of the original folio.
> Then putting back only the head page won't be enough.
> 
> Fix it by putting back all the folios on the list.

mm/migrate.c: In function 'migrate_misplaced_folio':
mm/migrate.c:2624:13: error: unused variable 'nr_pages' [-Werror=unused-variable]
 2624 |         int nr_pages = folio_nr_pages(folio);
      |             ^~~~~~~~

Worrisome.  Which kernel version was this tested against?

> Don't need to copy stable if this can still hit 6.10..  Only smoke tested.

Also worrisome.  Are we to take an only-smoke-tested patch which
doesn't apply to mainline and which doesn't compile on mm-unstable into
mainline based on "only smoke tested"?