mm/migrate: add folio_movable_ops()

folio_movable_ops() does the same as page_movable_ops() except uses folios
instead of pages.  This function will help make folio conversions in
migrate.c more readable.

Link: https://lkml.kernel.org/r/20230130214352.40538-3-vishal.moola@gmail.comSigned-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/migrate.h

@@ -122,6 +122,15 @@ static inline bool folio_test_movable(struct folio *folio)
 	return PageMovable(&folio->page);
 }
 
+static inline
+const struct movable_operations *folio_movable_ops(struct folio *folio)
+{
+	VM_BUG_ON(!__folio_test_movable(folio));
+
+	return (const struct movable_operations *)
+		((unsigned long)folio->mapping - PAGE_MAPPING_MOVABLE);
+}
+
 static inline
 const struct movable_operations *page_movable_ops(struct page *page)
 {

mm/migrate.c

@@ -990,7 +990,7 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
 			goto out;
 		}
 
-		mops = page_movable_ops(&src->page);
+		mops = folio_movable_ops(src);
 		rc = mops->migrate_page(&dst->page, &src->page, mode);
 		WARN_ON_ONCE(rc == MIGRATEPAGE_SUCCESS &&
 				!folio_test_isolated(src));