Re: [PATCH RFC 1/3] vfs: add copy_file_range syscall and vfs helper

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

On Tue, Apr 14, 2015 at 11:22:41AM -0700, Zach Brown wrote:
> On Tue, Apr 14, 2015 at 02:19:11PM -0400, J. Bruce Fields wrote:
> > On Tue, Apr 14, 2015 at 01:16:13PM -0400, Anna Schumaker wrote:
> > > On 04/14/2015 12:53 PM, Christoph Hellwig wrote:
> > > > On Sat, Apr 11, 2015 at 09:04:02AM -0400, Jeff Layton wrote:
> > > >> Yuck! How the heck do you clean up the mess if that happens? I
> > > >> guess you're just stuck redoing the copy with normal READ/WRITE?
> > > >>
> > > >> Maybe we need to have the interface return a hard error in that
> > > >> case and not try to give back any sort of offset?
> > > > 
> > > > The NFSv4.2 COPY interface is a train wreck.  At least for Linux I'd
> > > > expect us to simply ignore it and only implement my new CLONE
> > > > operation with sane semantics.  That is unless someone can show some
> > > > real life use case for the inter server copy, in which case we'll
> > > > have to deal with that mess.  But getting that one right at the VFS
> > > > level will be a nightmare anyway.
> > > > 
> > > > Make this a vote from me to not support partial copies and just
> > > > return and error in that case.
> > > 
> > > Agreed.  Looking at the v4.2 spec, COPY does take ca_consecutive and a
> > > ca_synchronous flags that let the client state if the copy should be
> > > done consecutively or synchronously.  I expected to always set
> > > consecutive to "true" for the Linux client.
> > 
> > That's supposed to mean results are well-defined in the partial-copy
> > case, but I think Christoph's suggesting eliminating the partial-copy
> > case entirely?
> > 
> > Which would be fine with me.
> > 
> > It might actually have been me advocating for partial copies.  But that
> > was only because a partial-copy-handling-loop seemed simpler to me than
> > progress callbacks if we were going to support long-running copies.
> > 
> > I'm happy enough not to have it at all.
> 
> Ah, OK, that's great news.
> 
> I thought at one point we were worried about very long running RPCs on
> the server.  Are we not worried about that now?
> 
> Is the client expected to cut the work up into arbitrarily managable
> chunks?  Is the server expected to fail COPY/CLONE requests that it
> thinks would take way too long?  Something else?

Christoph is proposing a CLONE rpc that's required to be atomic:

	https://tools.ietf.org/html/draft-ietf-nfsv4-minorversion2-35#section-15.13
	"The CLONE operation is atomic, that is either all changes or no
	changes are seen by the client or other clients."

So that couldn't be really long-running (or the server is nuts).

So that'd mean Anna would rip out the server-side copy loop and we'd
initially just support btrfs or whatever.

I mean the server-side copy loop may also be useful but I'm all for
wiring up the obvious case first.

--b.