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>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 975e899..160101a 100644 (file)
--- 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;
 }
 
 /*