mm: Add split_folio_to_list()

This is a convenience function; split_huge_page_to_list() can take
any page in a folio (and does so on purpose because that page will
be the one which keeps the refcount).  But it's convenient for the
callers to pass the folio instead of the first page in the folio.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

include/linux/huge_mm.h

@@ -483,6 +483,12 @@ static inline bool thp_migration_supported(void)
 }
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 
+static inline int split_folio_to_list(struct folio *folio,
+		struct list_head *list)
+{
+	return split_huge_page_to_list(&folio->page, list);
+}
+
 /**
  * thp_size - Size of a transparent huge page.
  * @page: Head page of a transparent huge page.

mm/vmscan.c

@@ -1715,16 +1715,16 @@ static unsigned int shrink_page_list(struct list_head *page_list,
 					 * tail pages can be freed without IO.
 					 */
 					if (!compound_mapcount(page) &&
-					    split_huge_page_to_list(page,
-								    page_list))
+					    split_folio_to_list(folio,
+								page_list))
 						goto activate_locked;
 				}
 				if (!add_to_swap(page)) {
 					if (!PageTransHuge(page))
 						goto activate_locked_split;
 					/* Fallback to swap normal pages */
-					if (split_huge_page_to_list(page,
-								    page_list))
+					if (split_folio_to_list(folio,
+								page_list))
 						goto activate_locked;
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 					count_vm_event(THP_SWPOUT_FALLBACK);
@@ -1740,7 +1740,7 @@ static unsigned int shrink_page_list(struct list_head *page_list,
 			}
 		} else if (unlikely(PageTransHuge(page))) {
 			/* Split file THP */
-			if (split_huge_page_to_list(page, page_list))
+			if (split_folio_to_list(folio, page_list))
 				goto keep_locked;
 		}