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

[Chris Mason]

On Wed, Sep 09, 2015 at 01:37:44PM -0700, Andy Lutomirski wrote:
> On Wed, Sep 9, 2015 at 1:09 PM, Chris Mason <address@hidden> 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 <address@hidden> 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?
> 
> My use case was archival.  I can reflink data between a working copy
> and some archived copy and then I can very efficiently tell if the
> working copy has been changed by checking if the reflink is still
> linked.
> 
> It would be even better if I could enumerate which parts of one file
> match which parts of another file.

Oh ok, we can do that pretty quickly with the btrfs searching ioctl
(just walk the items, really fast), but that's root only.

For a real interface maybe a btrfs specific ioctl to compare file
ranges.

-chris