From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757334AbeEJMit (ORCPT ); Thu, 10 May 2018 08:38:49 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:57482 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1757217AbeEJMis (ORCPT ); Thu, 10 May 2018 08:38:48 -0400 Date: Thu, 10 May 2018 14:38:43 +0200 From: Oleg Nesterov To: "Eric W. Biederman" Cc: Johannes Weiner , Michal Hocko , Kirill Tkhai , akpm@linux-foundation.org, peterz@infradead.org, viro@zeniv.linux.org.uk, mingo@kernel.org, paulmck@linux.vnet.ibm.com, keescook@chromium.org, riel@redhat.com, tglx@linutronix.de, kirill.shutemov@linux.intel.com, marcos.souza.org@gmail.com, hoeun.ryu@gmail.com, pasha.tatashin@oracle.com, gs051095@gmail.com, dhowells@redhat.com, rppt@linux.vnet.ibm.com, linux-kernel@vger.kernel.org, Balbir Singh , Tejun Heo Subject: Re: [PATCH] memcg: Replace mm->owner with mm->memcg Message-ID: <20180510123843.GB29222@redhat.com> References: <20180504142056.GA26151@redhat.com> <87r2mrh4is.fsf@xmission.com> <20180504145435.GA26573@redhat.com> <87y3gzfmjt.fsf@xmission.com> <20180504162209.GB26573@redhat.com> <871serfk77.fsf@xmission.com> <20180507143358.GA3071@redhat.com> <87vabyvnw0.fsf@xmission.com> <20180509144016.GA25742@redhat.com> <87vabwp5p6.fsf@xmission.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87vabwp5p6.fsf@xmission.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/09, Eric W. Biederman wrote: > > Oleg Nesterov writes: > > >> The patch does solve the issue. There should be nothing a userspace > >> process can observe that should tell it where in the middle of exec > >> such a migration happend so placing the migration at what from the > >> kernel's perspective might be technically later should not be a problem. > >> > >> If it is a problem the issue is that there is a way to observe the > >> difference. > > > > So. The task migrates from some MEMCG right after bprm_mm_init(). > > > > copy_strings() triggers OOM in MEMCG. This is quite possible, it can use a lot > > of memory and that is why we have acct_arg_size() to make these allocations > > visible to oom killer. > > > > task_in_mem_cgroup(MEMCG) returns false and oom killer has to kill another > > innocent process in MEMCG. > > > > Does this look like a way to observe the difference? > > Sort of. > > I don't know how the memcg gets away without migrating charges > when it migrates a process. With charges not being migrated > I don't think this is observable. Not sure I understand how this connects to accounting... But yes sure, with or without your change, mem_cgroup_move_task() obviously can't see the the nascent bprm->mm. I have no idea if this is important or not, and iiuc cgroup v2 doesn't even support ->move_charge_at_immigrate. As for accounting, I still think that it would be better to nullify ->memcg in mm_init_memcg(), simply because we can not initialize it properly, we can race with migration until exec_mmap/cgroup_post_fork which need to update ->memcg anyway. Yes, this means a special case in get_mem_cgroup_from_mm(). Oleg. > That does look like a real issue however. > > >> > Perhaps we can change get_mem_cgroup_from_mm() to use > >> > mem_cgroup_from_css(current, memory_cgrp_id) if mm->memcg == NULL? > >> > >> Please God no. Having any unnecessary special case is just going to > >> confuse people and cause bugs. > > > > To me the unnecessary special case is the new_mm->memcg which is used for > > accounting but doesn't follow migration till exec_mmap(). But I won't > > argue. > > Eric