mm: switch s_mem and slab_cache in struct page

This will allow us to store slub's counters in the same bits as slab's
s_mem.  slub now needs to set page->mapping to NULL as it frees the page,
just like slab does.

Link: http://lkml.kernel.org/r/20180518194519.3820-5-willy@infradead.orgSigned-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Acked-by: Christoph Lameter <cl@linux.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.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

@@ -83,7 +83,7 @@ struct page {
 		/* See page-flags.h for the definition of PAGE_MAPPING_FLAGS */
 		struct address_space *mapping;
 
-		void *s_mem;			/* slab first object */
+		struct kmem_cache *slab_cache;	/* SL[AU]B: Pointer to slab */
 		atomic_t compound_mapcount;	/* first tail page */
 		/* page_deferred_list().next	 -- second tail page */
 	};
@@ -194,7 +194,7 @@ struct page {
 		spinlock_t ptl;
 #endif
 #endif
-		struct kmem_cache *slab_cache;	/* SL[AU]B: Pointer to slab */
+		void *s_mem;			/* slab first object */
 	};
 
 #ifdef CONFIG_MEMCG

mm/slub.c

@@ -1695,6 +1695,7 @@ static void __free_slab(struct kmem_cache *s, struct page *page)
 	__ClearPageSlab(page);
 
 	page_mapcount_reset(page);
+	page->mapping = NULL;
 	if (current->reclaim_state)
 		current->reclaim_state->reclaimed_slab += pages;
 	memcg_uncharge_slab(page, order, s);