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

[Olga Kornievskaia <kolga@netapp.com>]

> On Mar 8, 2017, at 2:53 PM, J. Bruce Fields <bfields@fieldses.org> wrote:
> 
> 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?
> 

How would the application (cp) know when to call the clone_file_range and when to call copy_file_range?