mm: move _refcount out of struct page union

Keeping the refcount in the union only encourages people to put something
else in the union which will overlap with _refcount and eventually explode
messily.  pahole reports no fields change location.

Link: http://lkml.kernel.org/r/20180518194519.3820-7-willy@infradead.orgSigned-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm_types.h

@@ -113,7 +113,13 @@ struct page {
 		};
 	};
 
-	union {
+	union {		/* This union is 4 bytes in size. */
+		/*
+		 * If the page can be mapped to userspace, encodes the number
+		 * of times this page is referenced by a page table.
+		 */
+		atomic_t _mapcount;
+
 		/*
 		 * If the page is neither PageSlab nor mappable to userspace,
 		 * the value stored here may help determine what this page
@@ -124,21 +130,10 @@ struct page {
 
 		unsigned int active;		/* SLAB */
 		int units;			/* SLOB */
+	};
 
-		struct {			/* Page cache */
-			/*
-			 * Count of ptes mapped in mms, to show when
-			 * page is mapped & limit reverse map searches.
-			 */
-			atomic_t _mapcount;
-
-			/*
-			 * Usage count, *USE WRAPPER FUNCTION* when manual
-			 * accounting. See page_ref.h
-			 */
+	/* Usage count. *DO NOT USE DIRECTLY*. See page_ref.h */
 	atomic_t _refcount;
-		};
-	};
 
 	/*
 	 * WARNING: bit 0 of the first word encode PageTail(). That means