SLUB: Fix build breakage in linux/mm_types.h

On Wed, 6 Jul 2011, Jonathan Cameron wrote:

> Getting:
>
>   CHK     include/linux/version.h
>   CHK     include/generated/utsrelease.h
> make[1]: `include/generated/mach-types.h' is up to date.
>   CC      arch/arm/kernel/asm-offsets.s
> In file included from include/linux/sched.h:64:0,
>                  from arch/arm/kernel/asm-offsets.c:13:
> include/linux/mm_types.h:74:15: error: duplicate member '_count'
> make[1]: *** [arch/arm/kernel/asm-offsets.s] Error 1
> make: *** [prepare0] Error 2
>
> Issue looks to have been introduced by
>
>     mm: Rearrange struct page
>
> fc9bb8c768abe7ae10861c3510e01a95f98d5933
>
> Guessing it's a known issue, but just thought I'd flag it up in case
> it's something very specific about my build.
>
> gcc-2.6 armv7a
>
> Reverting that patch works, but given I don't know the history, I'm
> not proposing doing that in general!

Well _count exists in two unionized structs but always has the same offset
within the larger struct. Maybe ARM creates different offsets there for
some reason?

The following is a patch to restructure the union / structs combo in such
a way that only a single definition of _count

Reported-by: Jonathan Cameron <jic23@cam.ac.uk>
Tested-by: Piotr Hosowicz <piotr@hosowicz.com>
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

include/linux/mm_types.h

@@ -49,32 +49,27 @@ struct page {
 					 * see PAGE_MAPPING_ANON below.
 					 */
 	/* Second double word */
-	union {
-		struct {
+	struct {
+		union {
 			pgoff_t index;		/* Our offset within mapping. */
+			void *freelist;		/* slub first free object */
+		};
+
+		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 {
+			/* Used for cmpxchg_double in slub */
+			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;
-				};
 			};
 		};
+		atomic_t _count;		/* Usage count, see below. */
 	};
 
 	/* Third double word block */