Re: [PATCH] memcg: Remove memcg_cgroup::id from IDR on mem_cgroup_css_alloc() failure

[Michal Hocko <mhocko@kernel.org>]

On Fri 13-04-18 12:35:22, Kirill Tkhai wrote:
> On 13.04.2018 11:55, Michal Hocko wrote:
> > On Thu 12-04-18 17:52:04, Kirill Tkhai wrote:
> > [...]
> >> @@ -4471,6 +4477,7 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
> >>  
> >>  	return &memcg->css;
> >>  fail:
> >> +	mem_cgroup_id_remove(memcg);
> >>  	mem_cgroup_free(memcg);
> >>  	return ERR_PTR(-ENOMEM);
> >>  }
> > 
> > The only path which jumps to fail: here (in the current mmotm tree) is 
> > 	error = memcg_online_kmem(memcg);
> > 	if (error)
> > 		goto fail;
> > 
> > AFAICS and the only failure path in memcg_online_kmem
> > 	memcg_id = memcg_alloc_cache_id();
> > 	if (memcg_id < 0)
> > 		return memcg_id;
> > 
> > I am not entirely clear on memcg_alloc_cache_id but it seems we do clean
> > up properly. Or am I missing something?
> 
> memcg_alloc_cache_id() may allocate a lot of memory, in case of the system reached
> memcg_nr_cache_ids cgroups. In this case it iterates over all LRU lists, and double
> size of every of them. In case of memory pressure it can fail. If this occurs,
> mem_cgroup::id is not unhashed from IDR and we leak this id.

OK, my bad I was looking at the bad code path. So you want to clean up
after mem_cgroup_alloc not memcg_online_kmem. Now it makes much more
sense. Sorry for the confusion on my end.

Anyway, shouldn't we do the thing in mem_cgroup_free() to be symmetric
to mem_cgroup_alloc?
-- 
Michal Hocko
SUSE Labs