memcg, oom: lock mem_cgroup_print_oom_info

mem_cgroup_print_oom_info uses a static buffer (memcg_name) to store the
name of the cgroup.  This is not safe as pointed out by David Rientjes
because memcg oom is locked only for its hierarchy and nothing prevents
another parallel hierarchy to trigger oom as well and overwrite the
already in-use buffer.

This patch introduces oom_info_lock hidden inside
mem_cgroup_print_oom_info which is held throughout the function.  It makes
access to memcg_name safe and as a bonus it also prevents parallel memcg
ooms to interleave their statistics which would make the printed data hard
to analyze otherwise.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index d89ed16ab8117de13241e9edb460153123285ecc..9615407880ffeb290afceb7dfc3ac4ffc1c48597 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1647,13 +1647,13 @@ static void move_unlock_mem_cgroup(struct mem_cgroup *memcg,
  */
 void mem_cgroup_print_oom_info(struct mem_cgroup *memcg, struct task_struct *p)
 {
-       struct cgroup *task_cgrp;
-       struct cgroup *mem_cgrp;
        /*
-        * Need a buffer in BSS, can't rely on allocations. The code relies
-        * on the assumption that OOM is serialized for memory controller.
-        * If this assumption is broken, revisit this code.
+        * protects memcg_name and makes sure that parallel ooms do not
+        * interleave
         */
+       static DEFINE_SPINLOCK(oom_info_lock);
+       struct cgroup *task_cgrp;
+       struct cgroup *mem_cgrp;
        static char memcg_name[PATH_MAX];
        int ret;
        struct mem_cgroup *iter;
@@ -1662,6 +1662,7 @@ void mem_cgroup_print_oom_info(struct mem_cgroup *memcg, struct task_struct *p)
        if (!p)
                return;
 
+       spin_lock(&oom_info_lock);
        rcu_read_lock();
 
        mem_cgrp = memcg->css.cgroup;
@@ -1730,6 +1731,7 @@ done:
 
                pr_cont("\n");
        }
+       spin_unlock(&oom_info_lock);
 }
 
 /*