slab,slub: remove rcu_head size checks

rcu_head may now grow larger than list_head without affecting slab or
slub.

Link: http://lkml.kernel.org/r/20180518194519.3820-15-willy@infradead.org
Signed-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>

mm/slab.c

@@ -1235,8 +1235,6 @@ void __init kmem_cache_init(void)
 {
 	int i;
 
-	BUILD_BUG_ON(sizeof(((struct page *)NULL)->lru) <
-					sizeof(struct rcu_head));
 	kmem_cache = &kmem_cache_boot;
 
 	if (!IS_ENABLED(CONFIG_NUMA) || num_possible_nodes() == 1)

mm/slub.c

@@ -1686,17 +1686,9 @@ static void __free_slab(struct kmem_cache *s, struct page *page)
 	__free_pages(page, order);
 }
 
-#define need_reserve_slab_rcu						\
-	(sizeof(((struct page *)NULL)->lru) < sizeof(struct rcu_head))
-
 static void rcu_free_slab(struct rcu_head *h)
 {
-	struct page *page;
-
-	if (need_reserve_slab_rcu)
-		page = virt_to_head_page(h);
-	else
-		page = container_of((struct list_head *)h, struct page, lru);
+	struct page *page = container_of(h, struct page, rcu_head);
 
 	__free_slab(page->slab_cache, page);
 }
@@ -1704,19 +1696,7 @@ static void rcu_free_slab(struct rcu_head *h)
 static void free_slab(struct kmem_cache *s, struct page *page)
 {
 	if (unlikely(s->flags & SLAB_TYPESAFE_BY_RCU)) {
-		struct rcu_head *head;
-
-		if (need_reserve_slab_rcu) {
-			int order = compound_order(page);
-			int offset = (PAGE_SIZE << order) - s->reserved;
-
-			VM_BUG_ON(s->reserved != sizeof(*head));
-			head = page_address(page) + offset;
-		} else {
-			head = &page->rcu_head;
-		}
-
-		call_rcu(head, rcu_free_slab);
+		call_rcu(&page->rcu_head, rcu_free_slab);
 	} else
 		__free_slab(s, page);
 }
@@ -3583,9 +3563,6 @@ static int kmem_cache_open(struct kmem_cache *s, slab_flags_t flags)
 	s->random = get_random_long();
 #endif
 
-	if (need_reserve_slab_rcu && (s->flags & SLAB_TYPESAFE_BY_RCU))
-		s->reserved = sizeof(struct rcu_head);
-
 	if (!calculate_sizes(s, -1))
 		goto error;
 	if (disable_higher_order_debug) {