Re: [RFC 2/5] memcg: rework mem_cgroup_iter to use cgroup iterators

[Michal Hocko]

On Wed 14-11-12 11:10:52, Michal Hocko wrote:
> On Wed 14-11-12 09:20:03, KAMEZAWA Hiroyuki wrote:
> > (2012/11/14 0:30), Michal Hocko wrote:
> [...]
> > > @@ -1096,30 +1096,64 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> > >   			mz = mem_cgroup_zoneinfo(root, nid, zid);
> > >   			iter = &mz->reclaim_iter[reclaim->priority];
> > >   			spin_lock(&iter->iter_lock);
> > > +			last_visited = iter->last_visited;
> > >   			if (prev && reclaim->generation != iter->generation) {
> > > +				if (last_visited) {
> > > +					mem_cgroup_put(last_visited);
> > > +					iter->last_visited = NULL;
> > > +				}
> > >   				spin_unlock(&iter->iter_lock);
> > >   				return NULL;
> > >   			}
> > > -			id = iter->position;
> > >   		}
> > >   
> > >   		rcu_read_lock();
> > > -		css = css_get_next(&mem_cgroup_subsys, id + 1, &root->css, &id);
> > > -		if (css) {
> > > -			if (css == &root->css || css_tryget(css))
> > > -				memcg = mem_cgroup_from_css(css);
> > > -		} else
> > > -			id = 0;
> > > -		rcu_read_unlock();
> > > +		/*
> > > +		 * Root is not visited by cgroup iterators so it needs a special
> > > +		 * treatment.
> > > +		 */
> > > +		if (!last_visited) {
> > > +			css = &root->css;
> > > +		} else {
> > > +			struct cgroup *next_cgroup;
> > > +
> > > +			next_cgroup = cgroup_next_descendant_pre(
> > > +					last_visited->css.cgroup,
> > > +					root->css.cgroup);
> > 
> > Maybe I miss something but.... last_visited is holded by memcg's refcnt.
> > The cgroup pointed by css.cgroup is by cgroup's refcnt which can be freed
> > before memcg is freed and last_visited->css.cgroup is out of RCU cycle.
> > Is this safe ?
> 
> Good spotted. You are right. What I need to do is to check that the
> last_visited is alive and restart from the root if not. Something like
> the bellow (incremental patch on top of this one) should help, right?
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 30efd7e..c0a91a3 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1105,6 +1105,16 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
>  				spin_unlock(&iter->iter_lock);
>  				return NULL;
>  			}
> +			/*
> +			 * memcg is still valid because we hold a reference but
> +			 * its cgroup might have vanished in the meantime so
> +			 * we have to double check it is alive and restart the
> +			 * tree walk otherwise.
> +			 */
> +			if (last_visited && !css_tryget(&last_visited->css)) {
> +				mem_cgroup_put(last_visited);
> +				last_visited = NULL;
> +			}
>  		}
>  
>  		rcu_read_lock();
> @@ -1136,8 +1146,10 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
>  		if (reclaim) {
>  			struct mem_cgroup *curr = memcg;
>  
> -			if (last_visited)
> +			if (last_visited) {
> +				css_put(&last_visited->css);
>  				mem_cgroup_put(last_visited);
> +			}
>  
>  			if (css && !memcg)
>  				curr = mem_cgroup_from_css(css);

Now that I think about it again it seems that this is more complicated
than necessary. It should be sufficient to hold css' reference for the
iter->last_visited because this makes sure that the cgroup won't go
away same as mem_cgroup.
Memcg reference counting + css_tryget just makes the situation more
complicated because it forces us to retry the iteration on css_tryget
failure as the cgroup is gone already and we have no point to continue
other than start all over again. Which is, ehmm, _really_ ugly.

I will repost the updated version sometime this week after it passes
some testing.
-- 
Michal Hocko
SUSE Labs
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