Re: [PATCH v2 3/4] mm: thp: use folio_batch to handle THP splitting in deferred_split_scan()

[David Hildenbrand]

On 23.09.25 11:16, Qi Zheng wrote:
> From: Muchun Song <songmuchun@xxxxxxxxxxxxx>
>
> 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,
>        this lock protects the local list, not the split queue.
>
>     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@xxxxxxxxxxxxx>
> Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> ---

Nothing jumped at me

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers

David / dhildenb