[PATCH v4 5/6] mm/huge_memory: get frozen folio refcount with folio_expected_ref_count()

[Zi Yan]

Instead of open coding the refcount calculation, use
folio_expected_ref_count() to calculate frozen folio refcount.
Because:

1. __folio_split() does not split a folio with PG_private, so no elevated
   refcount from PG_private;
2. a frozen folio in __folio_split() is fully unmapped, so folio_mapcount()
   in folio_expected_ref_count() is always 0;
3. (mapping || swap_cache) ? folio_nr_pages(folio) is taken care of by
   folio_expected_ref_count() too.

Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
Acked-by: Balbir Singh <balbirs@xxxxxxxxxx>
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
---
 mm/huge_memory.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index d6ff5e8c89d7..4db67970ae69 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3737,6 +3737,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 	if (folio_ref_freeze(folio, 1 + extra_pins)) {
 		struct address_space *swap_cache = NULL;
 		struct lruvec *lruvec;
+		int expected_refs;
 
 		if (folio_order(folio) > 1 &&
 		    !list_empty(&folio->_deferred_list)) {
@@ -3800,11 +3801,8 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 		     new_folio = next) {
 			next = folio_next(new_folio);
 
-			folio_ref_unfreeze(
-				new_folio,
-				1 + ((mapping || swap_cache) ?
-					     folio_nr_pages(new_folio) :
-					     0));
+			expected_refs = folio_expected_ref_count(new_folio) + 1;
+			folio_ref_unfreeze(new_folio, expected_refs);
 
 			lru_add_split_folio(folio, new_folio, lruvec, list);
 
@@ -3834,8 +3832,8 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 		 * Otherwise, a parallel folio_try_get() can grab @folio
 		 * and its caller can see stale page cache entries.
 		 */
-		folio_ref_unfreeze(folio, 1 +
-			((mapping || swap_cache) ? folio_nr_pages(folio) : 0));
+		expected_refs = folio_expected_ref_count(folio) + 1;
+		folio_ref_unfreeze(folio, expected_refs);
 
 		unlock_page_lruvec(lruvec);
 
-- 
2.47.2