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Re: [libvirt] [RFC][PATCH] lxc: fix for ns cgroups subsystem



On Fri, May 08, 2009 at 08:34:12AM -0500, Serge E. Hallyn wrote:
> Quoting Ryota Ozaki (ozaki ryota gmail com):
> > Hi Serge,
> > 
> > On Fri, May 8, 2009 at 11:48 AM, Serge E. Hallyn <serue us ibm com> wrote:
> > > IIUC, the real problem is that src/cgroup.c assumes that the
> > > cgroup name should be $CGROUP_MOUNTPOINT/groupname.  But of
> > > course if the ns cgroup is enabled, then the unshare(CLONE_NEWNS)
> > > to create a new namespace in which to mount the new devpts
> > > locks the driver under $CGROUP_MOUNTPOINT/<pid_of_driver>/
> > > or somesuch.
> > >
> > > If this fixes the problem I have no objections, but it seems
> > > more fragile than perhaps trying to teach src/cgroup.c to
> > > consider it's current cgroup as a starting point.
> > 
> > hmm, I don't know why the assumption is bad and how the approach
> > you are suggesting helps the ns problem.
> 
> To be clear, the asssumption is that the driver starts in the
> root cgroup, i.e. it's pid is listed in $CGROUP_MOUNTPOINT/tasks.
> And that it can create $CGROUP_MOUNTPOINT/groupname and move
> itself into $CGROUP_MOUNTPOINT/groupname/tasks.
> 
> So, the assumption is bad because when the driver does a
> unshare(CLONE_NEWNS), it gets moved into $CGROUP_MOUNTPOINT/X,
> and after that can only move itself into
> $CGROUP_MOUNTPOINT/X/groupname.
> 
> Even with your patch, it's possible for the lxc driver to have
> been started under say $CGROUP_MOUNTPOINT/libvir or
> $CGROUP_MOUNTPOINT/<username> through libcgroup/PAM for instance,
> in which case your patch would be insufficient.

Indeed, we can't assume we're in the root group at any time. Our
general plan is that libvirt itself uses 3 levels of cgroup
starting at the cgroup that libvirtd was placed in by the admin
of the host, then a per-driver group, then per-guest groups

  $LIBVIRTD_ROOT_CGROUP
     |
     +- lxc
     |    |
     |    +- LXC-GUEST-1
     |    +- LXC-GUEST-2
     |    +- LXC-GUEST-3
     |    +- ...
     |
     +- qemu
          |
          +- QEMU-GUEST-1
          +- QEMU-GUEST-2
          +- QEMU-GUEST-3
          +- ...

$LIBVIRTD_ROOT_CGROUP, may be the actaul root mount point for
the cgroup controller in question, or it may be a sub-directory
that the admin chose to put it in. Also, if running libvirtd
as a normal user, the admin may have created per-user account
cgroups and so libvirtd would end up in there. So we have to
be prepared for anything wrt initial libvirtd cgroup root.

NB The code for putting QEMU in a cgroup isn't merged yet.


Daniel
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