[PATCH] memcg, oom: lock mem_cgroup_print_oom_info

[Michal Hocko]

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 although memcg oom is locked for its hierarchy 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 make 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.

Using the spinlock is OK here because this path is not hot and
meaningful data is much more important.

Reported-by: David Rientjes <rientjes@xxxxxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
---
 mm/memcontrol.c |   12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b73988a..d436316 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1575,13 +1575,13 @@ 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;
@@ -1590,6 +1590,7 @@ 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;
@@ -1658,6 +1659,7 @@ done:
 
 		pr_cont("\n");
 	}
+	spin_unlock(&oom_info_lock);
 }
 
 /*
-- 
1.7.10.4

-- 
Michal Hocko
SUSE Labs
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