[PATCH] mm/huge_memory: add folio_reset_partially_mapped()

[Kiryl Shutsemau]

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

__folio_unqueue_deferred_split() and __folio_freeze_and_split_unmapped()
both clear PG_partially_mapped and take the folio out of
MTHP_STAT_NR_ANON_PARTIALLY_MAPPED with the same five lines.

Move the block into folio_reset_partially_mapped() and call it from both
places.

The helper asserts what both callers rely on: the folio is frozen, so
deferred_split_folio() cannot set the flag again under it, and the folio
is already off the deferred split queue. The list check sits behind the
flag test because order-1 folios have no _deferred_list.

folio_order() is safe to use at this point in the split process: it
still shows the pre-split order.

Suggested-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
Assisted-by: Claude-Code:claude-fable-5-1
Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
---
 mm/huge_memory.c | 31 ++++++++++++++++++++-----------
 1 file changed, 20 insertions(+), 11 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index dd66c6ad5af1..23ef22c2b5db 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3972,6 +3972,24 @@ static unsigned int folio_cache_ref_count(const struct folio *folio)
 	return folio_nr_pages(folio);
 }
 
+static void folio_reset_partially_mapped(struct folio *folio)
+{
+	VM_WARN_ON_FOLIO(folio_ref_count(folio), folio);
+
+	if (!folio_test_partially_mapped(folio))
+		return;
+
+	/*
+	 * Order-1 folios have no _deferred_list. The flag is only ever set
+	 * on folios that do, so the list can be checked after the flag.
+	 */
+	VM_WARN_ON_FOLIO(!list_empty(&folio->_deferred_list), folio);
+
+	folio_clear_partially_mapped(folio);
+	mod_mthp_stat(folio_order(folio),
+		      MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1);
+}
+
 static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int new_order,
 					     struct page *split_at, struct xa_state *xas,
 					     struct address_space *mapping, bool do_lru,
@@ -3980,7 +3998,6 @@ 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);
 	int ret = 0;
 
 	VM_WARN_ON_ONCE(!mapping && end);
@@ -3998,11 +4015,7 @@ static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int n
 		 * 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);
-		}
+		folio_reset_partially_mapped(folio);
 
 		if (mapping) {
 			int nr = folio_nr_pages(folio);
@@ -4516,11 +4529,7 @@ bool __folio_unqueue_deferred_split(struct folio *folio)
 	memcg = folio_memcg(folio);
 	lru = list_lru_lock_irqsave(&deferred_split_lru, nid, &memcg, &flags);
 	if (__list_lru_del(&deferred_split_lru, lru, &folio->_deferred_list, nid)) {
-		if (folio_test_partially_mapped(folio)) {
-			folio_clear_partially_mapped(folio);
-			mod_mthp_stat(folio_order(folio),
-				      MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1);
-		}
+		folio_reset_partially_mapped(folio);
 		unqueued = true;
 	}
 	list_lru_unlock_irqrestore(lru, &flags);

base-commit: e3fc12b08aadde9cec7b3799ac0e0c9a1aa245c4
-- 
2.54.0