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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 3/10/26 11:54, 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@cmpxchg.org>
> Fixes: dafff3f4c850 ("mm: split underused THPs")
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---
> 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) &&

I don't think the folio_test_large_rmappable() check is required. Other
folios we migrate here would always have _deferred_list initialized but
unused.

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David