Re: [PATCH 7/8] cgroup: mount cgroupns-root when inside non-init cgroupns

["Serge E. Hallyn" <serge.hallyn@ubuntu.com>]

On Tue, Dec 01, 2015 at 11:46:49AM -0500, Tejun Heo wrote:
> Hey, Serge.
> 
> On Mon, Nov 30, 2015 at 10:07:04PM -0600, Serge E. Hallyn wrote:
> > So actually the way the code is now, the first mount cannot
> > be done from a non-init user namespace; and kernfs_obtain_root()
> > is only called from non-init user namespace.  So can we assume
> > that the root dentry will be instantiated?  (or can it get
> > evicted?)
> > 
> > If we can assume that then most of that fn can go away.
> 
> The v2 hierarchy is always mounted and non-init ns shouldn't be able
> to create new v1 hierarchies, so the root dentry should always be
> there.

I mispoke before though - it's not the hierarchy's root dentry,
but rather a dentry for a descendent cgroup which will become the
root dentry for the new superblock.  We do know that there must be
a css_set with a cgroup.  I'm still trying to track down whether
that cgrou's inode's dentry can ever be flushed.  I would think
not but am not sure.