[patch 08/10] mm: memcontrol: do not acquire page_cgroup lock for kmem pages

[Johannes Weiner <hannes@cmpxchg.org>]

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>
---
 mm/memcontrol.c | 21 +++++++--------------
 1 file changed, 7 insertions(+), 14 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f71eec597001..cd024dfb039a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3455,12 +3455,13 @@ 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)
@@ -3470,19 +3471,11 @@ 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);
+	memcg = pc->mem_cgroup;
+	pc->flags = 0;
 
 	/*
 	 * We trust that only if there is a memcg associated with the page, it
-- 
1.9.3