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

[Wei Yang]

On Wed, Oct 15, 2025 at 02:35:32PM +0800, 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,
>      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@xxxxxxxxxxxxx>
>Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
>Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
>Acked-by: David Hildenbrand <david@xxxxxxxxxx>
>Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
>---
> 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.

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

-- 
Wei Yang
Help you, Help me