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>

1 files changed, 10 insertions, 15 deletions

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 3d76a433d52..6dc6413f5de 100644
--- a/include/linux/mm_types.h
+++ b/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 */