linux-kernel - Re: [PATCH RFC 1/3] vfs: add copy_file

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Message-ID: <20150414181911.GA2080@fieldses.org>
Date:	Tue, 14 Apr 2015 14:19:11 -0400
From:	bfields@...ldses.org (J. Bruce Fields)
To:	Anna Schumaker <Anna.Schumaker@...app.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Jeff Layton <jlayton@...chiereds.net>,
	Trond Myklebust <trond.myklebust@...marydata.com>,
	Zach Brown <zab@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
	linux-btrfs@...r.kernel.org,
	Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
	linux-scsi@...r.kernel.org
Subject: Re: [PATCH RFC 1/3] vfs: add copy_file_range syscall and vfs helper

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.

--b.
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