[Devel] Re: LXC container, unmounting unneeded mount points (from the container namespace)

[Michael H. Warfield]

On Mon, 2010-01-11 at 22:54 -0500, Michael H. Warfield wrote: 
> Added lxc-devel to the cc list since this seems to be more appropriate
> over there (maybe) and I've just joined that one as well.
> 
> On Fri, 2010-01-08 at 15:35 +0100, Daniel Lezcano wrote: 
> > Ciprian Dorin, Craciun wrote:
> > > On Fri, Jan 8, 2010 at 4:17 PM, Daniel Lezcano <daniel.lezcano at free.fr> wrote:
> > >   
> > >> Ciprian Dorin, Craciun wrote:
> > >>     
> > >>>    Hy all!
> > >>>
> > >>>    Today I've started to play with the LXC containers, and I've got a
> > >>> question: when starting a container (with it's own mount point
> > >>> namespace), and I do a `cat /proc/mounts` I also see the mount points
> > >>> from my host system.
> > >>>
> > >>>    So the question is: how can I force `lxc-create` to remove any
> > >>> uneeded mount points (maybe all)?
> > >>>
> > >>>       
> > >> Committed today :)
> > >>
> > >> http://git.lab.toulouse-stg.fr.ibm.com/?p=lxc.git;a=commit;h=ed83715df7666879116d1657b1dd54a8fc6513f6
> > >>     

> > Oops, this one:

> > http://lxc.git.sourceforge.net/git/gitweb.cgi?p=lxc/lxc;a=commit;h=bf601689a9e0cea1ceaf17e4f7f853f5392c2827

> Nice.  This is EXACTLY what I've been looking for as well.  Only problem
> is...  It didn't work.  It complains about:

> lxc-start: could not umount 1 mounts"

> Then terminates saying it can't start the container.

> When I throw in some debugging prints, I find out that it's complaining
> that it can't umount {pivotdir}/dev.  Commented out the return -1's in
> that routine so it starts the container anyways and jump into the
> running container and I discover that {pivotdir}/dev/shm is still
> mounted even though it thinks it umounted it.  I can see from the
> debugging prints that it seems to have successfully umount it (the
> debugging message indicates that it umounted it - so it saw it had it
> and tried and got a good return from the attempt) but it's apparently
> still mount.  If I umount that {pivotdir}/dev/shm inside the container,
> I can then umount {pivotdir}/dev and {pivotdir}. 

> Tried it both by defining lxc.pivotdir and by letting it choose a
> temporary one.  Tried two different containers.  Same result.

:

Crud...  I always forget to mention SOMETHING...

On the host system /dev/shm is a tmpfs file system.

From mount:

tmpfs on /dev/shm type tmpfs (rw)

It's in my /etc/fstab like this:

tmpfs  /dev/shm  tmpfs   defaults        0 0

Seems to be that way on all my Fedora and CentOS systems.  I can umount
that in the host system (doesn't SEEM to do any harm) and then lxc-start
works just fine and can pivot the root and unmount everything.  Seems to
be something to do with tmpfs.

> Of course we also have the strange "rootfs" entry there as well.

> Host system is Fedora 12.
> Host kernel is: 2.6.31.9-174.fc12.i686
> Guest system in each case is CentOS 5.4
> 
> Regards,
> Mike

Mike
-- 
Michael H. Warfield (AI4NB) | (770) 985-6132 |  mhw at WittsEnd.com
   /\/\|=mhw=|\/\/          | (678) 463-0932 |  http://www.wittsend.com/mhw/
   NIC whois: MHW9          | An optimist believes we live in the best of all
 PGP Key: 0x674627FF        | possible worlds.  A pessimist is sure of it!
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