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

[Zi Yan]

On 10 Mar 2026, at 6:54, 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@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(+)
>
LGTM.

Acked-by: Zi Yan <ziy@xxxxxxxxxx>

Best Regards,
Yan, Zi