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

[Zi Yan <ziy@nvidia.com>]

On 1 Apr 2026, at 15:21, Zi Yan wrote:

> On 1 Apr 2026, at 9:10, Lance Yang wrote:
>
>> From: Lance Yang <lance.yang@linux.dev>
>>
>> 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.
>>
>> Link: https://syzkaller.appspot.com/bug?extid=a7067a757858ac8eb085
>> Fixes: 8a8ca142a488 ("mm: migrate: requeue destination folio on deferred split queue")
>> Reported-by: syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com
>> Closes: https://lore.kernel.org/linux-mm/69ccb65b.050a0220.183828.003a.GAE@google.com/
>> Cc: <stable@vger.kernel.org>
>> Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>> ---
>>
>> [ Backport note ]
>> This patch is a follow-up fix for 8a8ca142a488 ("mm: migrate: requeue
>> destination folio on deferred split queue"), which is currently only in
>> mm-stable, and should be backported together with it.
>>
>> Credit for this fix goes to David, thanks!
>>
>>  mm/huge_memory.c | 12 +++++++-----
>>  mm/migrate.c     | 18 +++++++++---------
>>  2 files changed, 16 insertions(+), 14 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index ff9a42abd1b6..ac6d823e351f 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -4558,7 +4558,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>>  				goto next;
>>  		}
>>  		if (!folio_trylock(folio))
>> -			goto next;
>> +			goto requeue;
>>  		if (!split_folio(folio)) {
>>  			did_split = true;
>>  			if (underused)
>> @@ -4569,11 +4569,13 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>>  next:
>>  		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.  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.
>>  		 */
>
> Should the sentence
> “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.”
> be moved to below label next?
>
> Since “thp_underused() returns false” is describing “if (!underused) goto next”,
> “split_folio() succeeds” is describing “did_split == true in the if”,
> “split_folio() fails in the case it was underused” is describing
> “did_split == false and !folio_test_partially_mapped(folio) in the if”.
>
> The first sentence matches the goto requeue for folio_trylock().

Hi Andrew,

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.


From 6ebeca9f7215cb91905d3f49385dbbafce5a80c2 Mon Sep 17 00:00:00 2001
From: Zi Yan <ziy@nvidia.com>
Date: Wed, 1 Apr 2026 18:52:43 -0400
Subject: [PATCH] move the comment.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 mm/huge_memory.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index ac6d823e351ff..970e077019b75 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -4567,15 +4567,18 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
 		}
 		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:
 		/*
-		 * Add back partially mapped folios, or underused folios
-		 * that we could not lock this round.  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.
+		 * 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)) {
-- 
2.53.0



>
> Otherwise, LGTM.
>
> Acked-by: Zi Yan <ziy@nvidia.com>
>
>>  		fqueue = folio_split_queue_lock_irqsave(folio, &flags);
>>  		if (list_empty(&folio->_deferred_list)) {
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 05cb408846f2..8a64291ab5b4 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1385,6 +1385,15 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
>>  	if (rc)
>>  		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
>> @@ -1401,15 +1410,6 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
>>  	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);
>> -- 
>> 2.49.0
>
>
> Best Regards,
> Yan, Zi


Best Regards,
Yan, Zi