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>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 0b8df1f2b0a363c782d2cf3895ae3b0e4875853f..ace67fc1d097009e63eb729e6c045cf812acde0e 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3458,12 +3458,13 @@ 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)
@@ -3473,19 +3474,11 @@ 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