[patch 10/12] mm: memcontrol: do not acquire page_cgroup lock for kmem pages

Johannes Weiner
2014-06-16T19:54:30Z

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 1cde6e2b33d9..764e182ccde3 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3414,12 +3414,13 @@ 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)
@@ -3429,19 +3430,11 @@ 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
--
2.0.0