Re: [PATCH 4/4] quota: add generic subtree quota support

[Christoph Hellwig <hch@infradead.org>]

On Tue, Feb 16, 2010 at 11:56:44AM +0300, Dmitry Monakhov wrote:
> what do you mean by "slightly limited" ?
> I dont now xfs project-id feature very well.
> I cant find good explanation of this feature(except man pages).
> Can you please post main design assumptions.

The only difference from your subtree quota is that multiple subtrees
can belong to a project.

XFS project quotas work the following way:

 - an inode can be marked as belonging to a project, in which case a
   32bit project ID is stored in the inode
 - a flag on the inode is set to inherit the project ID
 - adding links from outside the subtree is not allowed

quota uses the project ID for accounting, similar to the user/group id.

One other small difference is that going over the project quota does
not return EDQUOT but ENOSPC.

> In fact i use get_id in order to support second-level quota feature
> *Second-level quota*
>  In order to isolate user/group quota in one subtree from other subtree
>  we have to remap quota id similar to:
>    quota_uid = (subtree_id << 32) | uid;

> > And yes, fs-specific quota interfaces are an utter nightmare, speaking
> > as the person fixing all this crap up right now.
> I've prepare patches against ext4 because:

It's perfectly fine to impement it for one filesystem, the important bit
is to make sure we have consistant interfaces for it in generic code.
I don't think having a get_id callback to allow fs-specific mappings is
a good idea for that.