Re: [PATCH v3] mm: migrate: requeue destination folio on deferred split queue

[Johannes Weiner]

On Thu, Mar 12, 2026 at 03:47:23AM -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")
> Acked-by: Zi Yan <ziy@xxxxxxxxxx>
> Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>