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

[Wei Yang <richard.weiyang@gmail.com>]

On Fri, Nov 07, 2025 at 10:51:15AM +0800, Qi Zheng wrote:
>
>
>On 11/6/25 10:52 PM, Wei Yang wrote:
>> 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>
>> > ---
>> > mm/huge_memory.c | 87 +++++++++++++++++++++++-------------------------
>> > 1 file changed, 41 insertions(+), 46 deletions(-)
>> > 
>> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> > index a68f26547cd99..e850bc10da3e2 100644
>> > --- a/mm/huge_memory.c
>> > +++ b/mm/huge_memory.c
>> > @@ -3782,21 +3782,22 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>> > 		struct lruvec *lruvec;
>> > 		int expected_refs;
>> > 
>> > -		if (folio_order(folio) > 1 &&
>> > -		    !list_empty(&folio->_deferred_list)) {
>> > -			ds_queue->split_queue_len--;
>> > +		if (folio_order(folio) > 1) {
>> > +			if (!list_empty(&folio->_deferred_list)) {
>> > +				ds_queue->split_queue_len--;
>> > +				/*
>> > +				 * Reinitialize page_deferred_list after removing the
>> > +				 * page from the split_queue, otherwise a subsequent
>> > +				 * split will see list corruption when checking the
>> > +				 * page_deferred_list.
>> > +				 */
>> > +				list_del_init(&folio->_deferred_list);
>> > +			}
>> > 			if (folio_test_partially_mapped(folio)) {
>> > 				folio_clear_partially_mapped(folio);
>> > 				mod_mthp_stat(folio_order(folio),
>> > 					      MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1);
>> > 			}
>> > -			/*
>> > -			 * Reinitialize page_deferred_list after removing the
>> > -			 * page from the split_queue, otherwise a subsequent
>> > -			 * split will see list corruption when checking the
>> > -			 * page_deferred_list.
>> > -			 */
>> > -			list_del_init(&folio->_deferred_list);
>> 
>> @Andrew
>> 
>> Current mm-new looks not merge the code correctly?
>> 
>> The above removed code is still there.
>> 
>> @Qi
>> 
>> After rescan this, I am confused about this code change.
>> 
>> The difference here is originally it would check/clear partially_mapped if
>> folio is on a list. But now we would do this even folio is not on a list.
>> 
>> If my understanding is correct, after this change, !list_empty() means folio
>> is on its ds_queue. And there are total three places to remove it from
>> ds_queue.
>> 
>>    1) __folio_unqueue_deferred_split()
>>    2) deferred_split_scan()
>>    3) __folio_split()
>> 
>> In 1) and 2) we all clear partially_mapped bit before removing folio from
>> ds_queue, this means if the folio is not on ds_queue in __folio_split(), it is
>> not necessary to check/clear partially_mapped bit.
>
>In deferred_split_scan(), if folio_try_get() succeeds, then only the
>folio will be removed from ds_queue, but not clear partially_mapped.
>

Hmm... you are right. Sorry for the trouble.

>> 
>> Maybe I missed something, would you mind correct me on this?
>> 

-- 
Wei Yang
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