[PATCH v4 06/16] mm: Move _refcount out of struct page union

[Matthew Wilcox <willy@infradead.org>]

From: Matthew Wilcox <mawilcox@microsoft.com>

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.

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 include/linux/mm_types.h | 25 ++++++++++---------------
 1 file changed, 10 insertions(+), 15 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 23378a789af4..9828cd170251 100644
--- a/include/linux/mm_types.h
+++ b/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
@@ -124,22 +130,11 @@ 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
-			 */
-			atomic_t _refcount;
-		};
 	};
 
+	/* Usage count. *DO NOT USE DIRECTLY*. See page_ref.h */
+	atomic_t _refcount;
+
 	/*
 	 * WARNING: bit 0 of the first word encode PageTail(). That means
 	 * the rest users of the storage space MUST NOT use the bit to
-- 
2.17.0