linux-kernel - Re: New copyfile system call

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Date:	Thu, 21 Feb 2013 14:24:49 -0800
From:	Zach Brown <zab@...hat.com>
To:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	Ric Wheeler <rwheeler@...hat.com>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Chris L. Mason" <clmason@...ionio.com>,
	Christoph Hellwig <hch@...radead.org>,
	Alexander Viro <aviro@...hat.com>,
	"Martin K. Petersen" <mkp@....net>, Hannes Reinecke <hare@...e.de>,
	Joel Becker <jlbec@...lplan.org>
Subject: Re: New copyfile system call - discuss before LSF?

On Thu, Feb 21, 2013 at 08:50:27PM +0000, Myklebust, Trond wrote:
> On Thu, 2013-02-21 at 21:00 +0100, Paolo Bonzini wrote:
> > Il 21/02/2013 15:57, Ric Wheeler ha scritto:
> > >>>
> > >> sendfile64() pretty much already has the right arguments for a
> > >> "copyfile", however it would be nice to add a 'flags' parameter: the
> > >> NFSv4.2 version would use that to specify whether or not to copy file
> > >> metadata.
> > > 
> > > That would seem to be enough to me and has the advantage that it is an
> > > relatively obvious extension to something that is at least not totally
> > > unknown to developers.
> > > 
> > > Do we need more than that for non-NFS paths I wonder? What does reflink
> > > need or the SCSI mechanism?
> > 
> > For virt we would like to be able to specify arbitrary block ranges.
> > Copying an entire file helps some copy operations like storage
> > migration.  However, it is not enough to convert the guest's offloaded
> > copies to host-side offloaded copies.
> 
> So how would a system call based on sendfile64() plus my flag parameter
> prevent an underlying implementation from meeting your criterion?

If I'm guessing correctly, sendfile64()+flags would be annoying because
it's missing an out_fd_offset.  The host will want to offload the
guest's copies by calling sendfile on block ranges of a guest disk image
file that correspond to the mappings of the in and out files in the
guest.

You could make it work with some locking and out_fd seeking to set the
write offset before calling sendfile64()+flags, but ugh.

 ssize_t sendfile(int out_fd, int in_fd, off_t in_offset, off_t
                  out_offset, size_t count, int flags);

That seems closer.

We might also want to pre-emptively offer iovs instead of offsets,
because that's the very first thing that's going to be requested after
people prototype having to iterate calling sendfile() for each
contiguous copy region. 

- z
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