Avoid duplicate _count variables in page_struct

Restructure the union / struct cascade in struct page so that
we only have one definition of _count.

Tested-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

include/linux/mm_types.h

@@ -49,30 +49,31 @@ struct page {
 					 * see PAGE_MAPPING_ANON below.
 					 */
 	/* Second double word */
-	union {
-		struct {
+	struct {
+		union {
 			pgoff_t index;		/* Our offset within mapping. */
-			atomic_t _mapcount;	/* Count of ptes mapped in mms,
+			void *freelist;		/* slub first free object */
+		};
+
+		union {
+			/* Used for cmpxchg_double in slub */
+			unsigned long counters;
+
+			struct {
+
+				union {
+					atomic_t _mapcount;	/* Count of ptes mapped in mms,
 							 * to show when page is mapped
 							 * & limit reverse map searches.
 							 */
-			atomic_t _count;		/* Usage count, see below. */
-		};
 
-		struct {			/* SLUB cmpxchg_double area */
-			void *freelist;
-			union {
-				unsigned long counters;
-				struct {
-					unsigned inuse:16;
-					unsigned objects:15;
-					unsigned frozen:1;
-					/*
-					 * Kernel may make use of this field even when slub
-					 * uses the rest of the double word!
-					 */
-					atomic_t _count;
+					struct {
+						unsigned inuse:16;
+						unsigned objects:15;
+						unsigned frozen:1;
+					};
 				};
+				atomic_t _count;		/* Usage count, see below. */
 			};
 		};
 	};