Re: [RFC v1 01/19] fs: Don't copy beyond the end of the file

["J. Bruce Fields" <bfields@fieldses.org>]

On Wed, Mar 08, 2017 at 12:32:12PM -0500, Olga Kornievskaia wrote:
> 
> > On Mar 8, 2017, at 12:25 PM, Christoph Hellwig <hch@infradead.org>
> > wrote:
> > 
> > On Wed, Mar 08, 2017 at 12:05:21PM -0500, J. Bruce Fields wrote:
> >> Since copy isn't atomic that check is never going to be reliable.
> > 
> > That's true for everything that COPY does.  By that logic we should
> > not implement it at all (a logic that I'd fully support)
> 
> If you were to only keep CLONE then you’d lose a huge performance gain
> you get from server-to-server COPY. 

Yes.  Also, I think copy-like copy implementations have reasonable
semantics that are basically the same as read:

	- copy can return successfully with less copied than requested.
	- it's fine for the copied range to start and/or end past end of
	  file, it'll just return a short read.
	- A copy of more than 0 bytes returning 0 means you're at end of
	  file.

The particular problem here is that that doesn't fit how clone works at
all.

It feels like what happened is that copy_file_range() was made mainly
for the clone case, with the idea that copy might be reluctantly
accepted as a second-class implementation.

But the performance gain of copy offload is too big to just ignore, and
in fact it's what copy_file_range does on every filesystem but btrfs and
ocfs2 (and maybe cifs?), so I don't think we can just ignore it.

If we had separate copy_file_range and clone_file_range, I *think* it
could all be made sensible.  Am I missing something?

--b.