Re: [RFC] vfs generic subtree support

[Dmitry Monakhov <dmonakhov@openvz.org>]

Al Viro <viro@ZenIV.linux.org.uk> writes:

> On Tue, Feb 16, 2010 at 01:52:43PM +0300, Dmitry Monakhov wrote:
>> Initially i've posted RFC patch-set which add subtree support for ext4.
>> http://marc.info/?l=linux-ext4&m=126563931215496&w=2
>> But in fact this is rather generic feature which may be implemented 
>> in vfs layer similar to namespace or security feature.
>> 
>> A subtree of a directory tree T is a tree consisting of a directory
>> (the subtree root) in T and all of its descendants in T.
>> 
>> Subtree assumptions:
>> * Each inode has subtree id (this id is stored inside inode).
>> * Subtree id is inherent from parent directory if corresponding flag is set
>> * Inode can not belongs to different subtrees
>> 
>> i've ommit subtree_type feature in order to simplify brief explanation.
>> 
>> Subtree with id == 0 has special meaning. It may contains roots of
>> other subtrees (this feature is used for cross subtree renames)
>> This feature has much in common with XFS project_id.
>
> 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.
>
> We already can mount arbitrary subtrees of directory tree from given fs,
> as many times as we want and in as many places as we like.  With each
> mountpoint acting as barrier to link() and rename().