memcg: fix bad behavior in use_hierarchy file

I have an application that does the following:

* copy the state of all controllers attached to a hierarchy
* replicate it as a child of the current level.

I would expect writes to the files to mostly succeed, since they are
inheriting sane values from parents.

But that is not the case for use_hierarchy.  If it is set to 0, we succeed
ok.  If we're set to 1, the value of the file is automatically set to 1 in
the children, but if userspace tries to write the very same 1, it will
fail.  That same situation happens if we set use_hierarchy, create a
child, and then try to write 1 again.

Now, there is no reason whatsoever for failing to write a value that is
already there.  It doesn't even match the comments, that states:

 /* If parent's use_hierarchy is set, we can't make any modifications
  * in the child subtrees...

since we are not changing anything.

So test the new value against the one we're storing, and automatically
return 0 if we're not proposing a change.

Signed-off-by: Glauber Costa <glommer@parallels.com>
Cc: Dhaval Giani <dhaval.giani@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Ying Han <yinghan@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 55a85e1a342f..6d3dd54ec429 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3764,6 +3764,10 @@ static int mem_cgroup_hierarchy_write(struct cgroup *cont, struct cftype *cft,
                 parent_memcg = mem_cgroup_from_cont(parent);
 
         cgroup_lock();
+
+        if (memcg->use_hierarchy == val)
+                goto out;
+
         /*
          * If parent's use_hierarchy is set, we can't make any modifications
          * in the child subtrees. If it is unset, then the change can
@@ -3780,6 +3784,8 @@ static int mem_cgroup_hierarchy_write(struct cgroup *cont, struct cftype *cft,
                         retval = -EBUSY;
         } else
                 retval = -EINVAL;
+
+out:
         cgroup_unlock();
 
         return retval;