[Devel] Re: [PATCH] fix bad behavior in use_hierarchy file

[Michal Hocko]

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 <glommer at parallels.com>
> CC: Dhaval Giani <dhaval.giani at gmail.com>
> CC: Michal Hocko <mhocko at suse.cz>
> CC: Kamezawa Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
> CC: Johannes Weiner <hannes at cmpxchg.org>

One comment bellow...
Acked-by: Michal Hocko <mhocko at suse.cz>

> ---
>  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.

>  	/*
>  	 * 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