linux-kernel - Re: New copyfile system call

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Message-ID: <20130331212354.GA5240@dcvr.yhbt.net>
Date:	Sun, 31 Mar 2013 21:23:54 +0000
From:	Eric Wong <normalperson@...t.net>
To:	Pavel Machek <pavel@....cz>
Cc:	"Myklebust, Trond" <Trond.Myklebust@...app.com>,
	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?

Pavel Machek <pavel@....cz> wrote:
> Eric Wong wrote:
> > [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)
> 
> Could library be created to make it less annoying to use, and harder
> to misuse?

Maybe, but getting people to use the library would be the hard, too.
And a library would not reduce syscalls in the common case.

We already have current->splice_pipe for sendfile, so maybe splice can
be taught to transparently use that when neither FD is a pipe.

I also think a SPLICE_F_DONTWAIT flag might be necessary.  It would be a
superset of SPLICE_F_NONBLOCK, but also act like MSG_DONTWAIT for the
non-pipe socket.

> splice man page does not mention pipe size limit...

It probably should.  I think I discovered it by using it many years ago
and burned it into my mind.
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