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 14:06:40, Kirill Tkhai wrote:
> On 13.04.2018 14:02, Michal Hocko wrote:
> > 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?
> 
> We can't, since it's called from mem_cgroup_css_free(), which doesn't have a deal
> with idr freeing. All the asymmetry, we see, is because of the trick to unhash ID
> earlier, then from mem_cgroup_css_free().

Are you sure. It's been some time since I've looked at the quite complex
cgroup tear down code but from what I remember, css_free is called on
the css release (aka when the reference count drops to zero). mem_cgroup_id_put_many
seems to unpin the css reference so we should have idr_remove by the
time when css_free is called. Or am I still wrong and should go over the
brain hurting cgroup removal code again?
-- 
Michal Hocko
SUSE Labs