From mboxrd@z Thu Jan 1 00:00:00 1970 From: Kirill Tkhai Subject: Re: [PATCH] memcg: Remove memcg_cgroup::id from IDR on mem_cgroup_css_alloc() failure Date: Fri, 13 Apr 2018 14:06:40 +0300 Message-ID: <06931a83-91d2-3dcf-31cf-0b98d82e957f@virtuozzo.com> References: <152354470916.22460.14397070748001974638.stgit@localhost.localdomain> <20180413085553.GF17484@dhcp22.suse.cz> <20180413110200.GG17484@dhcp22.suse.cz> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=virtuozzo.com; s=selector1; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version; bh=AaUgDC00FLvyQ3pzyQXgO91RzHg7U56nDGU1G31jvHY=; b=KmIWX0y38lYl53Oz75w7Uekh85pkjMpqmM39Nl5VXAtV2ELYbeypRCIJpkT3NgTV4PJ6tj0OBs8HG3YmYY4uZ+5/HdqnYMp/m/sw3xnmhqiT4PTJD2+qB4DM9QlVm0ctJ/NMtBZlA6Y0sl+sm6x9YtRvo/1zQ5aPIg0RFmNrdho= In-Reply-To: <20180413110200.GG17484@dhcp22.suse.cz> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" To: Michal Hocko Cc: akpm@linux-foundation.org, hannes@cmpxchg.org, vdavydov.dev@gmail.com, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org 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