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

[Usama Arif <usama.arif@linux.dev>]

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@cmpxchg.org>
>> Fixes: dafff3f4c850 ("mm: split underused THPs")
>> Signed-off-by: Usama Arif <usama.arif@linux.dev>
>> ---
>>  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.


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);