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

[Chris Mason <clm@fb.com>]

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 <darrick.wong@oracle.com> 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()?

Is that the end goal to the sharing check?

-chris