From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f200.google.com (mail-wr0-f200.google.com [209.85.128.200]) by kanga.kvack.org (Postfix) with ESMTP id 215006B0007 for ; Fri, 13 Apr 2018 07:54:56 -0400 (EDT) Received: by mail-wr0-f200.google.com with SMTP id q6so4795412wre.20 for ; Fri, 13 Apr 2018 04:54:56 -0700 (PDT) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id 34si638338edm.8.2018.04.13.04.54.54 for (version=TLS1 cipher=AES128-SHA bits=128/128); Fri, 13 Apr 2018 04:54:55 -0700 (PDT) Date: Fri, 13 Apr 2018 13:54:54 +0200 From: Michal Hocko Subject: Re: [PATCH] memcg: Remove memcg_cgroup::id from IDR on mem_cgroup_css_alloc() failure Message-ID: <20180413115454.GL17484@dhcp22.suse.cz> References: <152354470916.22460.14397070748001974638.stgit@localhost.localdomain> <20180413085553.GF17484@dhcp22.suse.cz> <20180413110200.GG17484@dhcp22.suse.cz> <06931a83-91d2-3dcf-31cf-0b98d82e957f@virtuozzo.com> <20180413112036.GH17484@dhcp22.suse.cz> <6dbc33bb-f3d5-1a46-b454-13c6f5865fcd@virtuozzo.com> <20180413113855.GI17484@dhcp22.suse.cz> <8a81c801-35c8-767d-54b0-df9f1ca0abc0@virtuozzo.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8a81c801-35c8-767d-54b0-df9f1ca0abc0@virtuozzo.com> Sender: owner-linux-mm@kvack.org List-ID: To: Kirill Tkhai 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 Fri 13-04-18 14:49:32, Kirill Tkhai wrote: > On 13.04.2018 14:38, Michal Hocko wrote: > > On Fri 13-04-18 14:29:11, Kirill Tkhai wrote: [...] > >> mem_cgroup_id_put_many() unpins css, but this may be not the last reference to the css. > >> Thus, we release ID earlier, then all references to css are freed. > > > > Right and so what. If we have released the idr then we are not going to > > do that again in css_free. That is why we have that memcg->id.id > 0 > > check before idr_remove and memcg->id.id = 0 for the last memcg ref. > > count. So again, why cannot we do the clean up in mem_cgroup_free and > > have a less confusing code? Or am I just not getting your point and > > being dense here? > > We can, but mem_cgroup_free() called from mem_cgroup_css_alloc() is unlikely case. > The likely case is mem_cgroup_free() is called from mem_cgroup_css_free(), where > this idr manipulations will be a noop. Noop in likely case looks more confusing > for me. Well, I would really prefer to have _free being symmetric to _alloc so that you can rely that the full state is gone after _free is called. This confused the hell out of me. Because I _did_ expect that mem_cgroup_free would do that and so I was looking at completely different place. > Less confusing will be to move > > memcg->id.id = idr_alloc(&mem_cgroup_idr, NULL, > 1, MEM_CGROUP_ID_MAX, > GFP_KERNEL); > > into mem_cgroup_css_alloc(). How are you think about this? I would have to double check. Maybe it can be done on top. But for the actual fix and a stable backport potentially should be as clear as possible. Your original patch would be just fine but if I would prefer mem_cgroup_free for the symmetry. -- Michal Hocko SUSE Labs