Memory controller use rcu_read_lock() in mem_cgroup_cache_charge()

Hugh Dickins noticed that we were using rcu_dereference() without
rcu_read_lock() in the cache charging routine. The patch below fixes
this problem

Signed-off-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 3 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 975e89935d52..160101a05b4d 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -456,16 +456,20 @@ int mem_cgroup_charge(struct page *page, struct mm_struct *mm,
 int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
                                 gfp_t gfp_mask)
 {
+        int ret = 0;
         struct mem_cgroup *mem;
         if (!mm)
                 mm = &init_mm;
 
+        rcu_read_lock();
         mem = rcu_dereference(mm->mem_cgroup);
+        css_get(&mem->css);
+        rcu_read_unlock();
         if (mem->control_type == MEM_CGROUP_TYPE_ALL)
-                return mem_cgroup_charge_common(page, mm, gfp_mask,
+                ret = mem_cgroup_charge_common(page, mm, gfp_mask,
                                 MEM_CGROUP_CHARGE_TYPE_CACHE);
-        else
-                return 0;
+        css_put(&mem->css);
+        return ret;
 }
 
 /*