linux-kernel - Re: Mount namespace "dominant peer group"?

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Date:	Fri, 20 May 2016 16:15:12 -0700
From:	Ram Pai <linuxram@...ibm.com>
To:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:	Miklos Szeredi <mszeredi@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Al Viro <viro@...IV.linux.org.uk>
Subject: Re: Mount namespace "dominant peer group"?

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.


RP

> 
> Cheers,
> 
> Michael
> 
> -- 
> Michael Kerrisk
> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
> Linux/UNIX System Programming Training: http://man7.org/training/

-- 
Ram Pai