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
