Re: [PATCH] mm: migrate: requeue destination folio on deferred split queue

[Zi Yan]

On 6 Mar 2026, at 9:12, Usama Arif wrote:

> On 06/03/2026 13:49, David Hildenbrand (Arm) wrote:
>> On 3/6/26 14:35, Usama Arif wrote:
>>> During folio migration, __folio_migrate_mapping() removes the source
>>> folio from the deferred split queue, but the destination folio is never
>>> re-queued.  This causes underutilized THPs to escape the shrinker after
>>> NUMA migration, since they silently drop off the deferred split list.
>>>
>>> Fix this by calling deferred_split_folio() on the destination folio
>>> after a successful migration, for large rmappable folios.
>>>
>>> Reported-by: Johannes Weiner <hannes@xxxxxxxxxxx>
>>> Fixes: dafff3f4c850 ("mm: split underused THPs")
>>> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>
>>> ---
>>>  mm/migrate.c | 11 +++++++++++
>>>  1 file changed, 11 insertions(+)
>>>
>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>> index ece77ccb2ec0..98d0a594f7b7 100644
>>> --- a/mm/migrate.c
>>> +++ b/mm/migrate.c
>>> @@ -1393,6 +1393,17 @@ 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 a large folio that was on the queue. Without
>>> +	 * this, NUMA migration causes underutilized THPs to escape
>>> +	 * the shrinker since the source is unqueued in
>>> +	 * __folio_migrate_mapping() and the destination is never
>>> +	 * re-queued.
>>> +	 */
>>> +	if (folio_test_large(dst) && folio_test_large_rmappable(dst))
>>> +		deferred_split_folio(dst, false);
>>
>> Doesn't that mean that you will readd any large folios, even if already
>> previously taken off the list after scanning?
>>
>> So I am not sure if your "if the source was a large folio that was on
>> the queue." comment is accurate?
>>
>
> Yes you are right. How about something like below? We also won't need to check
> for anon and non-device folios with this as we only set the the flag if it was
> already on deferred_split list.

BTW, migrate_pages() tries to split partially mapped folios before migration[1],
so what remains in the deferred_list would be:

1. partially mapped but with a pin,
2. fully mapped but potentially underused.

I wonder if you want to do an underused scan before migration and try to split
underused THPs. Or to avoid this additional scan, find a way of detecting
zero pages at page copy time and split it after migration.

Anyway, it seems that all large folios are in this deferred_list. Maybe, like
David suggested in his LSFMM proposal, we should scan large folios on LRU lists
at reclaim time instead, since there is not much difference between deferred_list
and LRU lists right now.


[1] https://elixir.bootlin.com/linux/v6.19.3/source/mm/migrate.c#L1840

>
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index ece77ccb2ec0..9e0780d380e4 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1137,7 +1137,9 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
>  enum {
>         PAGE_WAS_MAPPED = BIT(0),
>         PAGE_WAS_MLOCKED = BIT(1),
> -       PAGE_OLD_STATES = PAGE_WAS_MAPPED | PAGE_WAS_MLOCKED,
> +       PAGE_WAS_ON_DEFERRED_SPLIT = BIT(2),
> +       PAGE_OLD_STATES = PAGE_WAS_MAPPED | PAGE_WAS_MLOCKED |
> +                         PAGE_WAS_ON_DEFERRED_SPLIT,
>  };
>
>  static void __migrate_folio_record(struct folio *dst,
> @@ -1373,6 +1375,15 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
>                 goto out_unlock_both;
>         }
>
> +       /*
> +        * Record whether the source folio is on the deferred split queue
> +        * before move_to_new_folio(), which unqueues it via
> +        * __folio_migrate_mapping().
> +        */
> +       if (folio_test_large(src) && folio_test_large_rmappable(src) &&
> +           !data_race(list_empty(&src->_deferred_list)))
> +               old_page_state |= PAGE_WAS_ON_DEFERRED_SPLIT;
> +
>         rc = move_to_new_folio(dst, src, mode);
>         if (rc)
>                 goto out;
> @@ -1393,6 +1404,15 @@ 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 record and check the state
> +        * from before move_to_new_folio().
> +        */
> +       if (old_page_state & PAGE_WAS_ON_DEFERRED_SPLIT)
> +               deferred_split_folio(dst, false);
> +
>  out_unlock_both:
>         folio_unlock(dst);
>         folio_set_owner_migrate_reason(dst, reason);


Best Regards,
Yan, Zi