linux-kernel - Re: New copyfile system call

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Date:	Sat, 23 Feb 2013 00:32:39 +0000
From:	Eric Wong <normalperson@...t.net>
To:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
Cc:	Zach Brown <zab@...hat.com>, 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?

"Myklebust, Trond" <Trond.Myklebust@...app.com> wrote:
> > -----Original Message-----
> > From: Zach Brown [mailto:zab@...hat.com]
> > Sent: Thursday, February 21, 2013 5:25 PM
> > To: Myklebust, Trond
> > Cc: Paolo Bonzini; Ric Wheeler; Linux FS Devel; linux-kernel@...r.kernel.org;
> > Chris L. Mason; Christoph Hellwig; Alexander Viro; Martin K. Petersen;
> > Hannes Reinecke; Joel Becker
> > 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.
> 
> psendfile() ?
> 
> I fully agree that sounds reasonable... Just being an ass. :-)

splice() already has offset for both fds and a flags arg:

       ssize_t splice(int fd_in, loff_t *off_in, int fd_out,
                      loff_t *off_out, size_t len, unsigned int flags);

The current downside is it requires one fd to be a pipe, so it's
just not very easy to use from my perspective[1].

> > 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.
> 
> vpsendfile() then? I agree that might be a little more future-proof. Particularly given that the underlying protocols tend to be fully asynchronous, and so it makes sense to queue up more than one copy at a time...

splicev() might be nice to have in that case, too.



[1] my splice() annoyances:
    * need to create/manage a pipe
    * copy size limited by pipe size
    * doesn't reduce userspace syscalls (just data copy overhead)
    * easy to misuse and starve with blocking sockets + big buffers
    * not many users, so bugs creep in (v3.7.8 was the first usable
      version of the 3.7 series for TCP sockets)
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