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

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

On Tue, Nov 24, 2015 at 12:16:10PM -0500, Tejun Heo wrote:
> Hello,
> 
> On Mon, Nov 16, 2015 at 01:51:44PM -0600, serge@hallyn.com wrote:
> > +struct dentry *kernfs_obtain_root(struct super_block *sb,
> > +				  struct kernfs_node *kn)
> > +{
> > +	struct dentry *dentry;
> > +	struct inode *inode;
> > +
> > +	BUG_ON(sb->s_op != &kernfs_sops);
> > +
> > +	/* inode for the given kernfs_node should already exist. */
> > +	inode = ilookup(sb, kn->ino);
> > +	if (!inode) {
> > +		pr_debug("kernfs: could not get inode for '");
> > +		pr_cont_kernfs_path(kn);
> > +		pr_cont("'.\n");
> > +		return ERR_PTR(-EINVAL);
> > +	}
> 
> Hmmm... but inode might not have been instantiated yet.  Why not use
> kernfs_get_inode()?
> 
> > +	/* instantiate and link root dentry */
> > +	dentry = d_obtain_root(inode);
> > +	if (!dentry) {
> > +		pr_debug("kernfs: could not get dentry for '");
> > +		pr_cont_kernfs_path(kn);
> > +		pr_cont("'.\n");
> > +		return ERR_PTR(-ENOMEM);
> > +	}
> > +
> > +	/* If this is a new dentry, set it up. We need kernfs_mutex because this
> > +	 * may be called by callers other than kernfs_fill_super. */
> 
> Formatting.
> 
> > +	mutex_lock(&kernfs_mutex);
> > +	if (!dentry->d_fsdata) {
> > +		kernfs_get(kn);
> > +		dentry->d_fsdata = kn;
> > +	} else {
> > +		WARN_ON(dentry->d_fsdata != kn);
> > +	}
> > +	mutex_unlock(&kernfs_mutex);
> > +
> > +	return dentry;
> > +}
> 
> Wouldn't it be simpler to walk dentry from kernfs root than
> duplicating dentry instantiation?

Sorry I don't think I'm following.  Are you suggesting walking the
kn->parent chain backward and doing d_lookup() at each point starting
with sb->s_root?