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