Re: [PATCH v5 3/4] mm: thp: use folio_batch to handle THP splitting in deferred_split_scan()

[Harry Yoo <harry.yoo@oracle.com>]

On Wed, Oct 15, 2025 at 02:35:32PM +0800, Qi Zheng wrote:
> From: Muchun Song <songmuchun@bytedance.com>
> 
> The maintenance of the folio->_deferred_list is intricate because it's
> reused in a local list.
> 
> Here are some peculiarities:
> 
>    1) When a folio is removed from its split queue and added to a local
>       on-stack list in deferred_split_scan(), the ->split_queue_len isn't
>       updated, leading to an inconsistency between it and the actual
>       number of folios in the split queue.
> 
>    2) When the folio is split via split_folio() later, it's removed from
>       the local list while holding the split queue lock. At this time,
>       the lock is not needed as it is not protecting anything.
> 
>    3) To handle the race condition with a third-party freeing or migrating
>       the preceding folio, we must ensure there's always one safe (with
>       raised refcount) folio before by delaying its folio_put(). More
>       details can be found in commit e66f3185fa04 ("mm/thp: fix deferred
>       split queue not partially_mapped"). It's rather tricky.
> 
> We can use the folio_batch infrastructure to handle this clearly. In this
> case, ->split_queue_len will be consistent with the real number of folios
> in the split queue. If list_empty(&folio->_deferred_list) returns false,
> it's clear the folio must be in its split queue (not in a local list
> anymore).
> 
> In the future, we will reparent LRU folios during memcg offline to
> eliminate dying memory cgroups, which requires reparenting the split queue
> to its parent first. So this patch prepares for using
> folio_split_queue_lock_irqsave() as the memcg may change then.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> Reviewed-by: Zi Yan <ziy@nvidia.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
> ---

Nothing unusual came up during my review, so:
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>

-- 
Cheers,
Harry / Hyeonggon