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

[Wei Yang]

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?

>+		src_deferred_split = true;
>+		src_partially_mapped = folio_test_partially_mapped(src);
>+	}
>+
> 	rc = move_to_new_folio(dst, src, mode);
> 	if (rc)
> 		goto out;
>@@ -1393,6 +1401,15 @@ 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 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);
>-- 
>2.47.3
>

-- 
Wei Yang
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