[PATCH] shrink struct page

Reduce the size of the pageframe for NR_CPUS>4, CONFIG_PREEMPT back to the
minimal size by unionising both ->private and ->mapping with the pagetable
lock.

It uses an anonymous struct and hence requires gcc-3.x.

Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/mm.h

@@ -223,17 +223,15 @@ struct page {
 					 * & limit reverse map searches.
 					 */
 	union {
+	    struct {
 		unsigned long private;		/* Mapping-private opaque data:
 					 	 * usually used for buffer_heads
 						 * if PagePrivate set; used for
-					 * swp_entry_t if PageSwapCache
-					 * When page is free, this indicates
-					 * order in the buddy system.
+						 * swp_entry_t if PageSwapCache.
+						 * When page is free, this
+						 * indicates order in the buddy
+						 * system.
 						 */
-#if NR_CPUS >= CONFIG_SPLIT_PTLOCK_CPUS
-		spinlock_t ptl;
-#endif
-	} u;
 		struct address_space *mapping;	/* If low bit clear, points to
 						 * inode address_space, or NULL.
 						 * If page mapped as anonymous
@@ -241,6 +239,11 @@ struct page {
 						 * it points to anon_vma object:
 						 * see PAGE_MAPPING_ANON below.
 						 */
+	    };
+#if NR_CPUS >= CONFIG_SPLIT_PTLOCK_CPUS
+	    spinlock_t ptl;
+#endif
+	};
 	pgoff_t index;			/* Our offset within mapping. */
 	struct list_head lru;		/* Pageout list, eg. active_list
 					 * protected by zone->lru_lock !
@@ -261,8 +264,8 @@ struct page {
 #endif /* WANT_PAGE_VIRTUAL */
 };
 
-#define page_private(page)		((page)->u.private)
-#define set_page_private(page, v)	((page)->u.private = (v))
+#define page_private(page)		((page)->private)
+#define set_page_private(page, v)	((page)->private = (v))
 
 /*
  * FIXME: take this include out, include page-flags.h in
@@ -815,7 +818,7 @@ static inline pmd_t *pmd_alloc(struct mm_struct *mm, pud_t *pud, unsigned long a
  * overflow into the next struct page (as it might with DEBUG_SPINLOCK).
  * When freeing, reset page->mapping so free_pages_check won't complain.
  */
-#define __pte_lockptr(page)	&((page)->u.ptl)
+#define __pte_lockptr(page)	&((page)->ptl)
 #define pte_lock_init(_page)	do {					\
 	spin_lock_init(__pte_lockptr(_page));				\
 } while (0)