Re: [PATCH mm-unstable 1/1] mm: fix deferred split queue races during migration

Lance Yang
2026-04-03T04:24:28Z

On 2026/4/2 07:19, Andrew Morton wrote:
> On Wed, 01 Apr 2026 18:55:48 -0400 Zi Yan <ziy@nvidia.com> wrote:
>
>> Can you apply the fixup below to move the comment? Lance told me he
>> would be away for a while, so he could not send a fixup to move
>> the comment.
>
> Thanks. I folded that into Lance's base patch so here's the whole
> thing:
>

Thank you all!
Lance

>
> From: Lance Yang <lance.yang@linux.dev>
> Subject: mm: fix deferred split queue races during migration
> Date: Wed, 1 Apr 2026 21:10:32 +0800
>
> migrate_folio_move() records the deferred split queue state from src and
> replays it on dst. Replaying it after remove_migration_ptes(src, dst, 0)
> makes dst visible before it is requeued, so a concurrent rmap-removal path
> can mark dst partially mapped and trip the WARN in deferred_split_folio().
>
> Move the requeue before remove_migration_ptes() so dst is back on the
> deferred split queue before it becomes visible again.
>
> Because migration still holds dst locked at that point, teach
> deferred_split_scan() to requeue a folio when folio_trylock() fails.
> Otherwise a fully mapped underused folio can be dequeued by the shrinker
> and silently lost from split_queue.
>
> [ziy@nvidia.com: move the comment]
> Link: https://lkml.kernel.org/r/FB71A764-0F10-4E5A-B4A0-BA4C7F138408@nvidia.com
> Link: https://syzkaller.appspot.com/bug?extid=a7067a757858ac8eb085
> Link: https://lkml.kernel.org/r/20260401131032.13011-1-lance.yang@linux.dev
> Fixes: 8a8ca142a488 ("mm: migrate: requeue destination folio on deferred split queue")
> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> Reported-by: syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/linux-mm/69ccb65b.050a0220.183828.003a.GAE@google.com/
> Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
> Acked-by: Zi Yan <ziy@nvidia.com>
> Cc: Alistair Popple <apopple@nvidia.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Barry Song <baohua@kernel.org>
> Cc: Byungchul Park <byungchul@sk.com>
> Cc: David Hildenbrand <david@kernel.org>
> Cc: Deepanshu Kartikey <kartikey406@gmail.com>
> Cc: Dev Jain <dev.jain@arm.com>
> Cc: Gregory Price <gourry@gourry.net>
> Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
> Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
> Cc: Lance Yang <lance.yang@linux.dev>
> Cc: Liam Howlett <liam.howlett@oracle.com>
> Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
> Cc: Matthew Brost <matthew.brost@intel.com>
> Cc: Nico Pache <npache@redhat.com>
> Cc: Rakie Kim <rakie.kim@sk.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Wei Yang <richard.weiyang@gmail.com>
> Cc: Ying Huang <ying.huang@linux.alibaba.com>
> Cc: Usama Arif <usama.arif@linux.dev>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
>
> mm/huge_memory.c | 15 ++++++++++-----
> mm/migrate.c | 18 +++++++++---------
> 2 files changed, 19 insertions(+), 14 deletions(-)
>
> --- a/mm/huge_memory.c~mm-fix-deferred-split-queue-races-during-migration
> +++ a/mm/huge_memory.c
> @@ -4542,7 +4542,7 @@ retry:
> goto next;
> }
> if (!folio_trylock(folio))
> - goto next;
> + goto requeue;
> if (!split_folio(folio)) {
> did_split = true;
> if (underused)
> @@ -4551,13 +4551,18 @@ retry:
> }
> folio_unlock(folio);
> next:
> + /*
> + * If thp_underused() returns false, or if split_folio()
> + * succeeds, or if split_folio() fails in the case it was
> + * underused, then consider it used and don't add it back to
> + * split_queue.
> + */
> if (did_split || !folio_test_partially_mapped(folio))
> continue;
> +requeue:
> /*
> - * Only add back to the queue if folio is partially mapped.
> - * If thp_underused returns false, or if split_folio fails
> - * in the case it was underused, then consider it used and
> - * don't add it back to split_queue.
> + * Add back partially mapped folios, or underused folios that
> + * we could not lock this round.
> */
> fqueue = folio_split_queue_lock_irqsave(folio, &flags);
> if (list_empty(&folio->_deferred_list)) {
> --- a/mm/migrate.c~mm-fix-deferred-split-queue-races-during-migration
> +++ a/mm/migrate.c
> @@ -1384,6 +1384,15 @@ static int migrate_folio_move(free_folio
> goto out;
>
> /*
> + * Requeue the destination folio on the deferred split queue if
> + * the source was on the queue. The source is unqueued in
> + * __folio_migrate_mapping(), so we recorded the state from
> + * before move_to_new_folio().
> + */
> + if (src_deferred_split)
> + deferred_split_folio(dst, src_partially_mapped);
> +
> + /*
> * When successful, push dst to LRU immediately: so that if it
> * turns out to be an mlocked page, remove_migration_ptes() will
> * automatically build up the correct dst->mlock_count for it.
> @@ -1399,15 +1408,6 @@ static int migrate_folio_move(free_folio
> if (old_page_state & PAGE_WAS_MAPPED)
> remove_migration_ptes(src, dst, 0);
>
> - /*
> - * Requeue the destination folio on the deferred split queue if
> - * the source was on the queue. The source is unqueued in
> - * __folio_migrate_mapping(), so we recorded the state from
> - * before move_to_new_folio().
> - */
> - if (src_deferred_split)
> - deferred_split_folio(dst, src_partially_mapped);
> -
> out_unlock_both:
> folio_unlock(dst);
> folio_set_owner_migrate_reason(dst, reason);
> _
>