Re: [PATCH 1/1] xfs: use reflink to assist unaligned copy_file_range calls

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Tue, Dec 01, 2020 at 10:02:06AM +0000, Christoph Hellwig wrote:
> On Mon, Nov 30, 2020 at 07:37:16PM -0800, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > Add a copy_file_range handler to XFS so that we can accelerate file
> > copies with reflink when the source and destination ranges are not
> > block-aligned.  We'll use the generic pagecache copy to handle the
> > unaligned edges and attempt to reflink the middle.
> 
> Isn't this something we could better handle in the VFS (or a generic
> helper) so that all file systems that support reflink could benefit?

Maybe.  I don't know if it's universally true that all filesystems
should fall back to reflinking the middle range and pagecache copying
the unaligned start/end.

The other thing is that xfs can easily support reflink on rtextsize > 1,
but that adds the requirement that we set i_blocksize to a larger value
than we do now... or find some other way to convey allocation unit size
to a generic version of the fallback.  OTOH that's pretty easy to do
from xfs_copy_file_range.

--D