From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757922Ab1KKPaT (ORCPT ); Fri, 11 Nov 2011 10:30:19 -0500 Received: from mx1.redhat.com ([209.132.183.28]:9385 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751284Ab1KKPaR (ORCPT ); Fri, 11 Nov 2011 10:30:17 -0500 Date: Fri, 11 Nov 2011 16:25:32 +0100 From: Oleg Nesterov To: Pavel Emelyanov Cc: Andrew Morton , Cyrill Gorcunov , Glauber Costa , Nathan Lynch , Tejun Heo , Linux Kernel Mailing List , Serge Hallyn , Daniel Lezcano Subject: Re: [PATCH 3/3] pids: Make it possible to clone tasks with given pids Message-ID: <20111111152532.GA22640@redhat.com> References: <4EBC0696.9030103@parallels.com> <4EBC06DB.3090202@parallels.com> <20111110184654.GA1006@redhat.com> <20111110185603.GA1757@redhat.com> <4EBCF4E7.4090002@parallels.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4EBCF4E7.4090002@parallels.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/11, Pavel Emelyanov wrote: > > >> Unless: you are using CLONE_NEWPID along with CLONE_CHILD_USEPIDS and > >> this child_tidptr array has only one pid (before zero pid). > > > > And, if you do clone(CLONE_NEWPID | CLONE_CHILD_USEPIDS), then > > new_ns->child_reaper == NULL (unless you pass "1" in child_tidptr[]) ? > > > >> So, could you please explain what I have missed? > > > > please ;) I guess I misread this patch completely. Help! > > This is how I plan to use this functionality. > > When creating an init of a container being restored I call > > pids[0] = 1; > pids[1] = 0; > > clone(CLONE_NEWPID | CLONE_CHILD_USEPIDS, &pids) Yep, this is clear. In this case everything works because the pid_ns has no pids (and thus ->last_pid == 0). But. Let me repeat the question, what if you do the same with pids[0] = 2 /* anything != 1 */ ? In this case we create the new pid_ns, but its ->child_reaper is NULL. Unless I missed something. > Then this created "init" task will have to read pids > from image files and call > > pids[0] = > pids[1] = 0 > > clone(CLONE_CHILD_USEPIDS, &pids); > > one by one. At this point the last_pid is still 0 Yes, understood. set_pidmap() bypasses the last_pid logic. Clever hack^Wtrick ;) May be this deserves a comment above "if (pid_ns->last_pid != 0)", and perhaps it would be more clean to do this check before anything else. Hmm. It seems, we can make a simpler patch to achieve the (roughly) same effect. Without touching copy_process/alloc_pid paths. What if we simply add PR_SET_LAST_PID? (or something else). In this case the new init (created normally) read the pids from image file and does prcrl(PR_SET_LAST_PID, pid-1) before the next fork. What do you think? Oleg.