mm/huge_memory: get frozen folio refcount with folio_expected_ref_count()

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.

Link: https://lkml.kernel.org/r/20250718023000.4044406-6-ziy@nvidia.com
Link: https://lkml.kernel.org/r/20250718183720.4054515-6-ziy@nvidia.com
Signed-off-by: Zi Yan <ziy@nvidia.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Acked-by: Balbir Singh <balbirs@nvidia.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Antonio Quartulli <antonio@mandelbit.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kirill A. Shutemov <k.shutemov@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Mathew Brost <matthew.brost@intel.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: x-y-z

mm/huge_memory.c

@@ -3731,6 +3731,7 @@ 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)) {
@@ -3794,11 +3795,8 @@ 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);
 
@@ -3828,8 +3826,8 @@ 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);