On Thu 23-10-14 14:19:29, Johannes Weiner wrote:
> On Thu, Oct 23, 2014 at 06:47:45PM +0200, Jan Kara wrote:
> > 'curr' pointer in task_in_mem_cgroup() can be NULL when we race with
> > somebody clearing task->mm. Check for it before dereferencing the
> > pointer.
>
> If task->mm is already NULL, we fall back to mem_cgroup_from_task(),
> which definitely returns a memcg unless you pass NULL in there. So I
> don't see how that could happen, and the NULL checks in the fallback
> branch as well as in __mem_cgroup_same_or_subtree seem bogus to me.
It came from 3a981f482cc2 (memcg: fix use_hierarchy css_is_ancestor oops
regression). I do not see mem_cgroup_same_or_subtree called from
page_referenced path so it is probably gone.
task_in_mem_cgroup is just confused because curr can never be NULL as
the task is never NULL.
---
>From e78d0ed29aa6732773109bd5ad69ac887d76bb78 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@xxxxxxx>
Date: Fri, 24 Oct 2014 10:46:08 +0200
Subject: [PATCH] memcg: Do not check NULL in __mem_cgroup_same_or_subtree
3a981f482cc2 (memcg: fix use_hierarchy css_is_ancestor oops
regression) has added NULL check into __mem_cgroup_same_or_subtree to
fix an oops in the page_referenced path. This is no longer needed
because all current paths are getting non-NULL memcg.
task_in_mem_cgroup is a little bit confused and assumes that
mem_cgroup_from_task may return NULL but this can never happen as the
task is alaways non-NULL. So get rid of that check as well.
Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
---
mm/memcontrol.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 3a203c7ec6c7..62064a5b496a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1339,7 +1339,7 @@ bool __mem_cgroup_same_or_subtree(const struct mem_cgroup *root_memcg,
{
if (root_memcg == memcg)
return true;
- if (!root_memcg->use_hierarchy || !memcg)
+ if (!root_memcg->use_hierarchy)
return false;
return cgroup_is_descendant(memcg->css.cgroup, root_memcg->css.cgroup);
}
@@ -1374,8 +1374,7 @@ bool task_in_mem_cgroup(struct task_struct *task,
*/
rcu_read_lock();
curr = mem_cgroup_from_task(task);
- if (curr)
- css_get(&curr->css);
+ css_get(&curr->css);
rcu_read_unlock();
}
/*
--
2.1.1
--
Michal Hocko
SUSE Labs
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