Re: [PATCH] memcg: don't do cleanup manually if mem_cgroup_css_online() fails

Michal Hocko
2013-04-02T13:32:27Z

On Tue 02-04-13 16:22:23, Glauber Costa wrote:
> On 04/02/2013 04:16 PM, Michal Hocko wrote:
> > On Tue 02-04-13 15:35:28, Li Zefan wrote:
> > [...]
> >> @@ -6247,16 +6247,7 @@ mem_cgroup_css_online(struct cgroup *cont)
> >>
> >> error = memcg_init_kmem(memcg, &mem_cgroup_subsys);
> >> mutex_unlock(&memcg_create_mutex);
> >> - if (error) {
> >> - /*
> >> - * We call put now because our (and parent's) refcnts
> >> - * are already in place. mem_cgroup_put() will internally
> >> - * call __mem_cgroup_free, so return directly
> >> - */
> >> - mem_cgroup_put(memcg);
> >> - if (parent->use_hierarchy)
> >> - mem_cgroup_put(parent);
> >> - }
> >> +
> >> return error;
> >> }
> >
> > The mem_cgroup_put(parent) part is incorrect because mem_cgroup_put goes
> > up the hierarchy already but I do not think mem_cgroup_put(memcg) should
> > go away as well. Who is going to free the last reference then?
> >
> > Maybe I am missing something but we have:
> > cgroup_create
> > css = ss->css_alloc(cgrp)
> > mem_cgroup_css_alloc
> > atomic_set(&memcg->refcnt, 1)
> > online_css(ss, cgrp)
> > mem_cgroup_css_online
> > error = memcg_init_kmem # fails
> > goto err_destroy
> > err_destroy:
> > cgroup_destroy_locked(cgrp)
> > offline_css
> > mem_cgroup_css_offline
> >
> > no mem_cgroup_put on the way.
> >
>
> static void mem_cgroup_css_free(struct cgroup *cont)
> {
> struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
>
> kmem_cgroup_destroy(memcg);
>
> mem_cgroup_put(memcg);
> }
>
> kernel/cgroup.c:
> err_free_all:
> for_each_subsys(root, ss) {
> if (cgrp->subsys[ss->subsys_id])
> ss->css_free(cgrp);
> }

But we do not get to that path after online_css fails because that one
jumps to err_destroy. So this is not it. Maybe css_free gets called from
cgroup_diput but I got lost in the indirection.
--
Michal Hocko
SUSE Labs