From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f199.google.com (mail-pf0-f199.google.com [209.85.192.199]) by kanga.kvack.org (Postfix) with ESMTP id 1E3EB6B0005 for ; Fri, 13 Apr 2018 07:06:50 -0400 (EDT) Received: by mail-pf0-f199.google.com with SMTP id v19so4613394pfn.7 for ; Fri, 13 Apr 2018 04:06:50 -0700 (PDT) Received: from EUR02-AM5-obe.outbound.protection.outlook.com (mail-eopbgr00131.outbound.protection.outlook.com. [40.107.0.131]) by mx.google.com with ESMTPS id p1si4415041pfi.355.2018.04.13.04.06.48 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Fri, 13 Apr 2018 04:06:49 -0700 (PDT) Subject: Re: [PATCH] memcg: Remove memcg_cgroup::id from IDR on mem_cgroup_css_alloc() failure References: <152354470916.22460.14397070748001974638.stgit@localhost.localdomain> <20180413085553.GF17484@dhcp22.suse.cz> <20180413110200.GG17484@dhcp22.suse.cz> From: Kirill Tkhai Message-ID: <06931a83-91d2-3dcf-31cf-0b98d82e957f@virtuozzo.com> Date: Fri, 13 Apr 2018 14:06:40 +0300 MIME-Version: 1.0 In-Reply-To: <20180413110200.GG17484@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: 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