[PATCHv2 7/7] cgroup: mount cgroupns-root when inside non-init cgroupns

[Serge E. Hallyn]

Quoting Andy Lutomirski (luto at amacapital.net):
> On Tue, Nov 4, 2014 at 5:46 AM, Tejun Heo <tj at kernel.org> wrote:
> > Hello, Aditya.
> >
> > On Mon, Nov 03, 2014 at 02:43:47PM -0800, Aditya Kali wrote:
> >> I agree that this is effectively bind-mounting, but doing this in kernel
> >> makes it really convenient for the userspace. The process that sets up the
> >> container doesn't need to care whether it should bind-mount cgroupfs inside
> >> the container or not. The tasks inside the container can mount cgroupfs on
> >> as-needed basis. The root container manager can simply unshare cgroupns and
> >> forget about the internal setup. I think this is useful just for the reason
> >> that it makes life much simpler for userspace.
> >
> > If it's okay to require userland to just do bind mounting, I'd be far
> > happier with that.  cgroup mount code is already overcomplicated
> > because of the dynamic matching of supers to mounts when it could just
> > have told userland to use bind mounting.  Doesn't the host side have
> > to set up some of the filesystem layouts anyway?  Does it really
> > matter that we require the host to set up cgroup hierarchy too?
> >
> 
> Sort of, but only sort of.
> 
> You can create a container by unsharing namespaces, mounting
> everything, and then calling pivot_root.  But this is unpleasant
> because of the strange way that pid namespaces work -- you generally
> have to fork first, so this gets tedious.  And it doesn't integrate
> well with things like fstab or other container-side configuration
> mechanisms.
> 
> It's nicer if you can unshare namespaces, mount the bare minimum,
> pivot_root, and let the contained software do as much setup as
> possible.

Also, the bind-mount requires the container manager to know where
the guest distro will want the cgroups mounted.

-serge