From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5D23DC43466 for ; Mon, 21 Sep 2020 15:05:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1FDA720758 for ; Mon, 21 Sep 2020 15:05:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727584AbgIUPFC (ORCPT ); Mon, 21 Sep 2020 11:05:02 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:36539 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726471AbgIUPFB (ORCPT ); Mon, 21 Sep 2020 11:05:01 -0400 Received: from ip5f5af089.dynamic.kabel-deutschland.de ([95.90.240.137] helo=wittgenstein) by youngberry.canonical.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.86_2) (envelope-from ) id 1kKNN5-0007iO-G6; Mon, 21 Sep 2020 15:04:51 +0000 Date: Mon, 21 Sep 2020 17:04:50 +0200 From: Christian Brauner To: Michal Hocko Cc: Tejun Heo , Peter Xu , Linus Torvalds , Jason Gunthorpe , John Hubbard , Leon Romanovsky , Linux-MM , Linux Kernel Mailing List , "Maya B . Gokhale" , Yang Shi , Marty Mcfadden , Kirill Shutemov , Oleg Nesterov , Jann Horn , Jan Kara , Kirill Tkhai , Andrea Arcangeli , Christoph Hellwig , Andrew Morton Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification Message-ID: <20200921150450.3mjjb3p3jwgatn4v@wittgenstein> References: <20200917193824.GL8409@ziepe.ca> <20200918164032.GA5962@xz-x1> <20200921134200.GK12990@dhcp22.suse.cz> <20200921141830.GE5962@xz-x1> <20200921142834.GL12990@dhcp22.suse.cz> <20200921143847.GB4268@mtj.duckdns.org> <20200921144355.mrzc66lina3dkfjq@wittgenstein> <20200921145537.GM12990@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200921145537.GM12990@dhcp22.suse.cz> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 21, 2020 at 04:55:37PM +0200, Michal Hocko wrote: > On Mon 21-09-20 16:43:55, Christian Brauner wrote: > > On Mon, Sep 21, 2020 at 10:38:47AM -0400, Tejun Heo wrote: > > > Hello, > > > > > > On Mon, Sep 21, 2020 at 04:28:34PM +0200, Michal Hocko wrote: > > > > Fundamentaly CLONE_INTO_CGROUP is similar to regular fork + move to the > > > > target cgroup after the child gets executed. So in principle there > > > > shouldn't be any big difference. Except that the move has to be explicit > > > > and the the child has to have enough privileges to move itself. I am not > > > > > > Yeap, they're supposed to be the same operations. We've never clearly > > > defined how the accounting gets split across moves because 1. it's > > > inherently blurry and difficult 2. doesn't make any practical difference for > > > the recommended and vast majority usage pattern which uses migration to seed > > > the new cgroup. CLONE_INTO_CGROUP doesn't change any of that. > > > > > > > completely sure about CLONE_INTO_CGROUP model though. According to man > > > > clone(2) it seems that O_RDONLY for the target cgroup directory is > > > > sufficient. That seems much more relaxed IIUC and it would allow to fork > > > > into a different cgroup while keeping a lot of resources in the parent's > > > > proper. > > > > > > If the man page is documenting that, it's wrong. cgroup_css_set_fork() has > > > an explicit cgroup_may_write() test on the destination cgroup. > > > CLONE_INTO_CGROUP should follow exactly the same rules as regular > > > migrations. > > > > Indeed! > > The O_RDONLY mention on the manpage doesn't make sense but it is > > explained that the semantics are exactly the same for moving via the > > filesystem: > > OK, if the semantic is the same as for the task migration then I do not > see any (new) problems. Care to point me where the actual check is > enforced? For the migration you need a write access to cgroup.procs but > if the API expects directory fd then I am not sure how that would expose > the same behavior. kernel/cgroup/cgroup.c:cgroup_csset_fork() So there's which is the first check for inode_permission() essentially: /* * Verify that we the target cgroup is writable for us. This is * usually done by the vfs layer but since we're not going through * the vfs layer here we need to do it "manually". */ ret = cgroup_may_write(dst_cgrp, sb); if (ret) goto err; and what you're referring to is checked right after in: ret = cgroup_attach_permissions(cset->dfl_cgrp, dst_cgrp, sb, !(kargs->flags & CLONE_THREAD)); if (ret) goto err; which calls: ret = cgroup_procs_write_permission(src_cgrp, dst_cgrp, sb); if (ret) return ret; That should be what you're looking for. I've also added selftests as always that verify this behavior under: tools/testing/selftests/cgroup/ as soon as CLONE_INTO_CGROUP is detected on the kernel than all the usual tests are exercised using CLONE_INTO_CGROUP so we should've seen any regression hopefully. Thanks! Christian