From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl0-f71.google.com (mail-pl0-f71.google.com [209.85.160.71]) by kanga.kvack.org (Postfix) with ESMTP id 3F67A6B0009 for ; Fri, 13 Apr 2018 08:07:24 -0400 (EDT) Received: by mail-pl0-f71.google.com with SMTP id m6-v6so5784207pln.8 for ; Fri, 13 Apr 2018 05:07:24 -0700 (PDT) Received: from EUR01-HE1-obe.outbound.protection.outlook.com (mail-he1eur01on0133.outbound.protection.outlook.com. [104.47.0.133]) by mx.google.com with ESMTPS id e32-v6si5656808plb.135.2018.04.13.05.07.22 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Fri, 13 Apr 2018 05:07:23 -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> <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> <20180413115454.GL17484@dhcp22.suse.cz> From: Kirill Tkhai Message-ID: Date: Fri, 13 Apr 2018 15:07:14 +0300 MIME-Version: 1.0 In-Reply-To: <20180413115454.GL17484@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 14:54, Michal Hocko wrote: > 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. We definitely can move id allocation to mem_cgroup_css_alloc(), but this is really not for an easy fix, which will be backported to stable. Moving idr destroy to mem_cgroup_free() hides IDR trick. My IMHO it's less readable for a reader. The main problem is allocation asymmetric, and we shouldn't handle it on free path... Kirill