mm: memcontrol: do not acquire page_cgroup lock for kmem pages

Kmem page charging and uncharging is serialized by means of exclusive
access to the page.  Do not take the page_cgroup lock and don't set
pc->flags atomically.
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -3407,12 +3407,13 @@ void __memcg_kmem_commit_charge(struct page *page, struct mem_cgroup *memcg,
 		memcg_uncharge_kmem(memcg, PAGE_SIZE << order);
 		return;
 	}
-
+	/*
+	 * The page is freshly allocated and not visible to any
+	 * outside callers yet.  Set up pc non-atomically.
+	 */
 	pc = lookup_page_cgroup(page);
-	lock_page_cgroup(pc);
 	pc->mem_cgroup = memcg;
-	SetPageCgroupUsed(pc);
-	unlock_page_cgroup(pc);
+	pc->flags = PCG_USED;
 }
 
 void __memcg_kmem_uncharge_pages(struct page *page, int order)
@@ -3422,19 +3423,11 @@ void __memcg_kmem_uncharge_pages(struct page *page, int order)
 
 
 	pc = lookup_page_cgroup(page);
-	/*
-	 * Fast unlocked return. Theoretically might have changed, have to
-	 * check again after locking.
-	 */
 	if (!PageCgroupUsed(pc))
 		return;
 
-	lock_page_cgroup(pc);
-	if (PageCgroupUsed(pc)) {
 	memcg = pc->mem_cgroup;
-		ClearPageCgroupUsed(pc);
-	}
-	unlock_page_cgroup(pc);
+	pc->flags = 0;
 
 	/*
 	 * We trust that only if there is a memcg associated with the page, it