[PATCH 3/6] vfs: support FS_XFLAG_REFLINK and FS_XFLAG_COWEXTSIZE

[Darrick J. Wong]

On Mon, Sep 05, 2016 at 07:56:22AM -0700, Christoph Hellwig wrote:
> On Thu, Aug 25, 2016 at 04:31:00PM -0700, Darrick J. Wong wrote:
> > Introduce XFLAGs for the new XFS reflink inode flag and the CoW extent
> > size hint, and actually plumb the CoW extent size hint into the fsxattr
> > structure.
> 
> Just curious, but why would we even bother to expose the reflink flag
> to userspace?

So far I've put the reflink flag to use in xfs_scrub to look for
obvious signs of brokenness such as extents that overlap or have the
shared flag set but the inode flag is off; and to skip various kinds
of checks that don't have to happen when blocks don't overlap.

I doubt there's much of a use for the flag outside of the XFS utilities.
For a while I pondered only exposing the fsxattr flag if the caller had
CAP_SYS_ADMIN (the level of priviledge required to run scrub) but
decided that I wouldn't change the existing interface like that unless
I had a really good reason.

--D