Re: [PATCH 08/15] vfs: change clone and dedupe range function pointers to return bytes completed

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

On Sat, Oct 06, 2018 at 03:41:34AM -0700, Christoph Hellwig wrote:
> On Thu, Oct 04, 2018 at 05:45:35PM -0700, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > Change the clone_file_range and dedupe_file_range functions to return
> > the number of bytes they operated on.  This is the precursor to allowing
> > fs implementations to return short clone/dedupe results to the user,
> > which will enable us to obey resource limits in a graceful manner.
> 
> For clone this doesn't make any sense, as we don't allow 'short clones'.

That's correct.  The overall goal is that copy_file_range will be able
to call ->clone_file_range with a "I can handle a short clone" flag so
that the fs (and the prep functions) know that they're allowed to
shorten the request length, and we have a way to communicate the actual
results back to userspace.

Neither FS_IOC_CLONE nor FS_IOC_CLONERANGE will be able to take
advantage of this, of course.  They won't pass in the flag, and any
implementation that wants to shorten the length will pass back -EINVAL
instead.

--D