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
