Re: [PATCH v2] mm: migrate: requeue destination folio on deferred split queue

[David Hildenbrand (Arm)]

On 3/12/26 04:18, Wei Yang wrote:
> On Tue, Mar 10, 2026 at 03:54:19AM -0700, 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 recording whether the source folio was on the deferred split
>> queue and its partially mapped state before move_to_new_folio() unqueues
>> it, and re-queuing the destination folio after a successful migration if
>> it was.
>>
>> By the time migrate_folio_move() runs, partially mapped folios without a
>> pin have already been split by migrate_pages_batch().  So only two cases
>> remain on the deferred list at this point:
>>  1. Partially mapped folios with a pin (split failed).
>>  2. Fully mapped but potentially underused folios.
>> The recorded partially_mapped state is forwarded to deferred_split_folio()
>> so that the destination folio is correctly re-queued in both cases.
>>
>> Reported-by: Johannes Weiner <hannes@xxxxxxxxxxx>
>> Fixes: dafff3f4c850 ("mm: split underused THPs")
>> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>
>> ---
>> v1 -> v2:
>> - record whether source folio was on the deferred split queue before
>>  move_to_folio() (David)
>> - record partially mapped state and update commit message (Zi)
>> ---
>> mm/migrate.c | 17 +++++++++++++++++
>> 1 file changed, 17 insertions(+)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index ece77ccb2ec0..61013d258eb4 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1360,6 +1360,8 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
>> 	int rc;
>> 	int old_page_state = 0;
>> 	struct anon_vma *anon_vma = NULL;
>> +	bool src_deferred_split = false;
>> +	bool src_partially_mapped = false;
>> 	struct list_head *prev;
>>
>> 	__migrate_folio_extract(dst, &old_page_state, &anon_vma);
>> @@ -1373,6 +1375,12 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
>> 		goto out_unlock_both;
>> 	}
>>
>> +	if (folio_test_large(src) && folio_test_large_rmappable(src) &&
>> +	    !data_race(list_empty(&src->_deferred_list))) {
> 
> We usually check order > 1, before accessing _deferred_list, because it is in
> subpage 2.
> 
> I am not sure why we don't do it here. Do I miss something?

Valid point! non-anon folios could trigger that.

-- 
Cheers,

David