linux-kernel - Re: [RFC] extending splice for copy offloading

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Message-ID: <20130911211722.GA9725@dcvr.yhbt.net>
Date:	Wed, 11 Sep 2013 21:17:23 +0000
From:	Eric Wong <normalperson@...t.net>
To:	Zach Brown <zab@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-nfs@...r.kernel.org,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Bryan Schumaker <bjschuma@...app.com>,
	"Martin K. Petersen" <mkp@....net>, Jens Axboe <axboe@...nel.dk>,
	Mark Fasheh <mfasheh@...e.com>,
	Joel Becker <jlbec@...lplan.org>
Subject: Re: [RFC] extending splice for copy offloading

Zach Brown <zab@...hat.com> wrote:
> Towards the end of that thread Eric Wong asked why we didn't just
> extend splice.  I immediately replied with some dumb dismissive
> answer.  Once I sat down and looked at it, though, it does make a
> lot of sense.  So good job, Eric.  +10 Dummie points for me.

Thanks for revisiting that :>

> Some things to talk about:
> - I really don't care about the naming here.  If you do, holler.

Exposing "DIRECT" to userspace now might confuse users into expecting
O_DIRECT behavior.  I say this as an easily-confused user.

In the future, perhaps O_DIRECT behavior can become per-splice (instead
of just per-open) and can save SPLICE_F_DIRECT for that.

> - We might want different flags for file-to-file splicing and acceleration
> - We might want flags to require or forbid acceleration

> - We might want to provide all these flags to sendfile, too

Another syscall?  I prefer not.  Better to just maintain the sendfile
API as-is for compatibility reasons and nudge users towards splice.

> Thoughts?  Objections?

I'll try to test/comment more in a week or two (not much time for
computing until then).
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