Re: [PATCH] fix bad behavior in use_hierarchy file
From: Michal Hocko <hidden>
Date: 2012-06-25 12:08:27
Also in:
cgroups
On Mon 25-06-12 13:21:01, Glauber Costa wrote:
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. The following patch tests the new value against the one we're storing, and automatically return 0 if we're not proposing a change.
Fair enough.
Signed-off-by: Glauber Costa <redacted> CC: Dhaval Giani <redacted> CC: Michal Hocko <redacted> CC: Kamezawa Hiroyuki <redacted> CC: Johannes Weiner <hannes@cmpxchg.org>
One comment bellow... Acked-by: Michal Hocko <redacted>
quoted hunk ↗ jump to hunk
--- mm/memcontrol.c | 6 ++++++ 1 file changed, 6 insertions(+)diff --git a/mm/memcontrol.c b/mm/memcontrol.c index ac35bcc..cccebbc 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c@@ -3779,6 +3779,10 @@ 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; +
Why do you need cgroup_lock to check the value? Even if we have 2 CPUs racing (one trying to set to 0 other to 1 with use_hierarchy==0) then the "set to 0" operation might fail depending on who hits the cgroup_lock first anyway. So while this is correct I think there is not much point to take the global cgroup lock in this case.
quoted hunk ↗ jump to hunk
/* * 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@@ -3795,6 +3799,8 @@ static int mem_cgroup_hierarchy_write(struct cgroup *cont, struct cftype *cft, retval = -EBUSY; } else retval = -EINVAL; + +out: cgroup_unlock(); return retval;-- 1.7.10.2
-- Michal Hocko SUSE Labs SUSE LINUX s.r.o. Lihovarska 1060/12 190 00 Praha 9 Czech Republic -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>