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

[Kirill Tkhai]

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().

Kirill