[PATCH 2/5] mm/huge_memory: dequeue the deferred split after the split freeze

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>

__folio_freeze_and_split_unmapped() takes the deferred split list_lru lock
across the freeze. It is only there to stop deferred_split_scan() from
touching the folio under split.

With deferred_split_isolate() fixed, the workaround can be dropped.

Unqueue the folio after folio_ref_freeze(), the way
__folio_migrate_mapping() does: folio_unqueue_deferred_split() needs a
zero refcount and a memcg still set, and both hold there.

If the split is called from deferred_split_scan(), the unqueue is a
no-op -- the folio is already removed from the list. But
PG_partially_mapped is still set, so it has to be cleared here or
MTHP_STAT_NR_ANON_PARTIALLY_MAPPED never comes back down.

Assisted-by: Claude-Code:claude-opus-5
Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
---
 mm/huge_memory.c | 44 +++++++++++++-------------------------------
 1 file changed, 13 insertions(+), 31 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 6281ed993243..c84e8cbc986d 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3931,41 +3931,27 @@ static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int n
 	struct folio *end_folio = folio_next(folio);
 	struct folio *new_folio, *next;
 	int old_order = folio_order(folio);
-	struct list_lru_one *lru;
-	bool dequeue_deferred;
 	int ret = 0;
 
 	VM_WARN_ON_ONCE(!mapping && end);
-	/*
-	 * If this folio can be on the deferred split queue, lock out
-	 * the shrinker before freezing the ref. If the shrinker sees
-	 * a 0-ref folio, it assumes it beat folio_put() to the list
-	 * lock and must clean up the LRU state - the same dequeue we
-	 * will do below as part of the split.
-	 */
-	dequeue_deferred = folio_test_anon(folio) && old_order > 1;
-	if (dequeue_deferred) {
-		struct mem_cgroup *memcg;
 
-		rcu_read_lock();
-		memcg = folio_memcg(folio);
-		lru = list_lru_lock(&deferred_split_lru,
-				    folio_nid(folio), &memcg);
-	}
 	if (folio_ref_freeze(folio, folio_cache_ref_count(folio) + 1)) {
 		struct swap_cluster_info *ci = NULL;
 		struct lruvec *lruvec;
 
-		if (dequeue_deferred) {
-			__list_lru_del(&deferred_split_lru, lru,
-				       &folio->_deferred_list, folio_nid(folio));
-			if (folio_test_partially_mapped(folio)) {
-				folio_clear_partially_mapped(folio);
-				mod_mthp_stat(old_order,
-					MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1);
-			}
-			list_lru_unlock(lru);
-			rcu_read_unlock();
+		/* Take off the deferred split queue while frozen and memcg set */
+		folio_unqueue_deferred_split(folio);
+
+		/*
+		 * deferred_split_scan() takes the folio off the queue before it
+		 * splits it, so the unqueue above finds an empty list and
+		 * leaves PG_partially_mapped set.
+		 * Clear it here: the flag does not survive the split.
+		 */
+		if (folio_test_partially_mapped(folio)) {
+			folio_clear_partially_mapped(folio);
+			mod_mthp_stat(old_order,
+				      MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1);
 		}
 
 		if (mapping) {
@@ -4067,10 +4053,6 @@ static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int n
 		if (ci)
 			swap_cluster_unlock(ci);
 	} else {
-		if (dequeue_deferred) {
-			list_lru_unlock(lru);
-			rcu_read_unlock();
-		}
 		return -EAGAIN;
 	}
 
-- 
2.54.0