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

[Usama Arif]

On 22/06/2026 14:43, Wei Yang wrote:
> On Mon, Jun 22, 2026 at 11:16:39AM +0200, David Hildenbrand (Arm) wrote:
>> On 6/20/26 09:27, 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))) {
>>>> +		src_deferred_split = true;
>>>> +		src_partially_mapped = folio_test_partially_mapped(src);
>>>> +	}
>>>
>>> Hi, Usama
>>>
>>> I am afraid there maybe a race between migration and defer_split.
>>>
>>>                 A                              B
>>>   migrate_pages_batch                   deferred_split_scan
>>>     migrate_folio_unmap                   list_del_init(&folio->_deferred_list)
>>>       folio_lock/folio_trylock
>>>
>>>     migrate_folios_move
>>>       migrate_folio_move
>>>         list_empty(&src->_deferred_list)
>>>                                           folio_trylock()
>>>                                           requeue:
>>>
>>> In case list_empty() check happens after folio removed from defer_list but
>>> before requeued, we will miss this folio.
>>
>> deferred_split_isolate() would grab a reference through folio_try_get().
>>
>> How can we migrate a folio with a raised refcount?
>>
> 
> Thanks, I missed expected_refcount check in __migrate_folio().
> 

Thanks David for pointing it out! I have just started looking at the
mailing list for today :)

>> -- 
>> Cheers,
>>
>> David
>