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 15:07:14 +0300 Message-ID: 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> 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=k+Jf+4lfPoZ9VRTjT4YMRmtCoH7V0oksItuKWbYRUuE=; b=LcWbpCDaKsCIIWbhvj2uHZdmGs+pOyrSwI2X/xq9JBlEb/qL4LSHVX+rRUOB1rjLJfEcqAT4+7dHN5Ravgj9PGrVCvvDmf6aqV0nbhf9F3QxUCD8QIkjn5BN2LhA+e0FR7CRNkfSYCJSrMuLF3IT2ZjyPYv06iM1Z9Aspua/onM= In-Reply-To: <20180413115454.GL17484@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 14:54, 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