[RFC, PATCH 0/2] fiemap: filesystem free space mapping

[Christoph Hellwig]

On Thu, Oct 18, 2012 at 04:11:17PM +1100, Dave Chinner wrote:
> So, I was bored a few days ago, and I was sick of having to run
> xfs_db incorrectly report free space extents when the filesytem is
> mounted, so I decided to extend fiemap to export freespace mappings
> to userspace so I could get the information coherently through the
> mounted filesystem.
> 
> Yes, this could probably be considered interface abuse but, well, it
> was simple to do because extent mapping is exactly what fiemap is
> designed to do. Hence I didn't have to write new walkers/formatters
> and I was using code I knew worked correctly.

I think the right way to handle this is to introduce a new ioctl which
uses the same structures.  That way we have a reasonable interface,
without issue like which file does it need to be called on because the
VFS glue can turn it into a superblock op.