# Re: \[PATCH v1 0/8\] VFS: In-kernel copy system call

- From: Chris Mason \<hidden\>
- Date: 2015-09-09 20:38:05 
- List: linux-api
- Page: https://ratatoskr.run/linux-api/2015/09/99244
- Thread: https://ratatoskr.run/linux-api/2015/09/99244/t.md

```text
On Wed, Sep 09, 2015 at 04:26:58PM -0400, Trond Myklebust wrote:
> On Wed, Sep 9, 2015 at 4:09 PM, Chris Mason [ref] wrote:
> > On Tue, Sep 08, 2015 at 04:08:43PM -0700, Andy Lutomirski wrote:
> >> On Tue, Sep 8, 2015 at 3:39 PM, Darrick J. Wong [ref] wrote:
> >> > On Tue, Sep 08, 2015 at 02:45:39PM -0700, Andy Lutomirski wrote:
> >> >> What I meant by this was: if you ask for "regular copy", you may end
> >> >> up with a reflink anyway.  Anyway, how can you reflink a range and
> >> >> have the contents *not* be the same?
> >> >
> >> > reflink forcibly remaps fd_dest's range to fd_src's range.  If they didn't
> >> > match before, they will afterwards.
> >> >
> >> > dedupe remaps fd_dest's range to fd_src's range only if they match, of course.
> >> >
> >> > Perhaps I should have said "...if the contents are the same before the call"?
> >> >
> >>
> >> Oh, I see.
> >>
> >> Can we have a clean way to figure out whether two file ranges are the
> >> same in a way that allows false negatives?  I.e. return 1 if the
> >> ranges are reflinks of each other and 0 if not?  Pretty please?  I've
> >> implemented that in the past on btrfs by syncing the ranges and then
> >> comparing FIEMAP output, but that's hideous.
> >
> > I'd almost rather have a separate call, maybe unshare_file_range()?
> >
> 
> Doesn't it make more sense to put that functionality in fallocate()?

That works too, I'm just hoping to keep the copy_file_range stuff
simple.

-chris
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