[PATCH v4 15/16] slab,slub: Remove rcu_head size checks

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>

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

Signed-off-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
---
 mm/slab.c |  2 --
 mm/slub.c | 27 ++-------------------------
 2 files changed, 2 insertions(+), 27 deletions(-)

diff --git a/mm/slab.c b/mm/slab.c
index e387a17d6d56..e6ab1327db25 100644
--- a/mm/slab.c
+++ b/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)
diff --git a/mm/slub.c b/mm/slub.c
index 04625e3dab13..27cc2956acba 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1690,17 +1690,9 @@ 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);
 }
@@ -1708,19 +1700,7 @@ 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);
 }
@@ -3587,9 +3567,6 @@ 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) {
-- 
2.17.0