Re: [RFC] vfs generic subtree support

[Al Viro <viro@ZenIV.linux.org.uk>]

On Tue, Feb 16, 2010 at 03:37:58PM +0300, Dmitry Monakhov wrote:

> > Um.  Just how is it different from having normal subtrees mounted separately?
> > And what's the ID for?
> For example for quota needs. With subtree support we can account some
> subtree in to corresponding quota_subtree id.

What does that have to do with tree topology?  Having it inherited from
parent is fine, but the rest...  If you want to prevent renames/links
across an arbitrary subtree boundary, you can already have such policy
without any kernel changes; just mount them separately.  I'm afraid
I still don't get it...

I certainly see a point in having some kind of "quota group ID" assigned
to fs objects, but that seems to be completely independent from any
tree topology considerations.

Again, setting up a barrier is as simple as adding
	/foo/bar /foo/bar none bind,rw 0 0
in /etc/fstab or doing
	mount --bind /foo/bar /foo/bar
when (or after) you've mounted your fs.  Or
	for i in /foo/*; do mount --bind $i $i; done
if you want all top-level subdirectories in /foo to be barriers, etc.
Either will prevent objects from one subtree to be renamable/linkable
from another.  Can be arbitrary nested as well...

IOW, that looks like a trivially implemented policy that might or might not
be desirable for specific setup, but I don't see the reason to tie this quota
groups stuff to it or to its reimplementation...