From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752154AbcEUMtw (ORCPT ); Sat, 21 May 2016 08:49:52 -0400 Received: from mail-yw0-f194.google.com ([209.85.161.194]:36620 "EHLO mail-yw0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751251AbcEUMtu (ORCPT ); Sat, 21 May 2016 08:49:50 -0400 Subject: Re: Mount namespace "dominant peer group"? To: Ram Pai References: <20160520231512.GA9481@ram.oc3035372033.ibm.com> Cc: mtk.manpages@gmail.com, Miklos Szeredi , lkml , "Serge E. Hallyn" , Al Viro From: "Michael Kerrisk (man-pages)" Message-ID: <7a07c930-215b-bdcd-3b08-a5c12cffcfe3@gmail.com> Date: Sat, 21 May 2016 07:49:45 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.0 MIME-Version: 1.0 In-Reply-To: <20160520231512.GA9481@ram.oc3035372033.ibm.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello Ram, On 05/20/2016 06:15 PM, Ram Pai wrote: > On Fri, May 20, 2016 at 04:24:18PM -0500, Michael Kerrisk (man-pages) wrote: >> Hello Miklos, >> >> I'm working on some better documentation of mount namespaces, >> and there's a detail that puzzles me, and I hope you might be >> able to help, since you added the detail... >> >> In Documentation/filesystems/proc.txt there is this text in the >> description of /proc/PID/mountinfo: >> >> [[ >> Parsers should ignore all unrecognised optional fields. Currently the >> possible optional fields are: >> >> shared:X mount is shared in peer group X >> master:X mount is slave to peer group X >> propagate_from:X mount is slave and receives propagation from peer group X (*) >> unbindable mount is unbindable >> >> (*) X is the closest dominant peer group under the process's root. If >> X is the immediate master of the mount, or if there's no dominant peer >> group under the same root, then only the "master:X" field is present >> and not the "propagate_from:X" field. >> ]] >> >> What is a dominant peer group, as distinct from the immediate master? >> >> I can see in fs/proc_namespaces.c that there is this distinction made: >> >> [[ >> /* Tagged fields ("foo:X" or "bar") */ >> if (IS_MNT_SHARED(r)) >> seq_printf(m, " shared:%i", r->mnt_group_id); >> if (IS_MNT_SLAVE(r)) { >> int master = r->mnt_master->mnt_group_id; >> int dom = get_dominating_id(r, &p->root); >> seq_printf(m, " master:%i", master); >> if (dom && dom != master) >> seq_printf(m, " propagate_from:%i", dom); >> } >> ]] >> >> But I can't relate that to some user-space semantics. I suppose another >> way of asking my question is: how could I create a slave that is >> propagating from a peer group other than it's immediate master? > > It can happen if you have unmounted or privatised all your master mounts from the peer group. > > Eg: > > mount /dev/xyz /1 #creates a new mount > mount --make-private /1 #just make sure that it does not receive or send and propogation > mount --make-shared /1 #now make it shared. > mount --bind /1 /2 #create a peer /1 and /2 are peers > create a new fs-namespace. this new fs-namespace which will have /1' and /2'. /1 /2 /1' /2' are now all part of the same peergroup. > mount --make-slave /2 # this will make /2 a slave of the peer group that contains /1 /1' and /2' > umount /1 # we now have /2 which receives propagation from a peer group which does not have a representative in its fs-namespace. Thanks for the note. However, doing the above, I still do not see any mount being marked with 'propagate_from'. Perhaps I misunderstood your instructions above. Here's what I did: sh1# mount --make-private / # Make share everything is private... sh1# mount /dev/sdb6 /1 sh1# mount --make-private /1 sh1# mount --make-shared /1 sh1# mount --bind /1 /2 sh1# cat /proc/self/mountinfo | grep '/[12] ' | sed 's/ - .*//' 81 61 8:22 / /1 rw,relatime shared:1 82 61 8:22 / /2 rw,relatime shared:1 Then, at a second terminal, create a new mount NS: sh2# unshare -m --propagation unchanged sh sh2# cat /proc/self/mountinfo | grep '/[12] ' | sed 's/ - .*//' 169 132 8:22 / /1 rw,relatime shared:1 170 132 8:22 / /2 rw,relatime shared:1 Returning to the first terminal: sh1# mount --make-slave /2 sh1# umount /1 sh1# cat /proc/self/mountinfo | grep '/[12] ' | sed 's/ - .*//' 82 61 8:22 / /2 rw,relatime master:1 That is, we see /2 in the initial mount namespace is a slave but there is no 'propagate_from' tag. Did I miss something? Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/