[RFC][PATCH 00/12] Enhanced file stat system call

[Christoph Hellwig]

On Tue, Nov 24, 2015 at 09:48:22AM +0100, Martin Steigerwald wrote:
> It might be interesting for BTRFS as well, to be able to ask what amount of 
> free space there currently is *at* a given path. Cause with BTRFS and 
> Subvolumes this may differ between different paths.

We can handle this trivial with the current statfs interface.  Take a
look at xfs_fs_statfs and xfs_qm_statvfs.