linux-kernel - Re: splice between non-pipe (tcp socket) fds

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Message-ID: <4AB9CF44.1030301@ge.com>
Date:	Wed, 23 Sep 2009 09:33:24 +0200
From:	Enrik Berkhan <Enrik.Berkhan@...com>
To:	Марк Коренберг 
	<socketpair@...il.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: splice between non-pipe (tcp socket) fds

Марк Коренберг wrote:
> I want splice() to work with two tcp sockets().I think to implement
> this. Two questions:

This works already by introducing a pipe between the two:

while (0 < (len = splice(sock1, 0, pipe, 0, max_len, some_flags)))
   splice(pipe, 0, sock2, 0, len, some_flags);

Just like with read(2) and write(2), but without copy to/from userspace.

> 1. Is any work in this direction?
> 2. Why this functionality does not implemented now? any difficulties?
> 
> I think API should consist of 2 main functions:
> 1. detach available buffers from fd1 (and save info in some struct)
> 2. attach buffers to fd2 (from this struct)
> this functions should be called under some lock to guarantee
> atomicity. if something happen, return buffers back to original file.

What about full duplex?

> At now, I think that the most quick way to acheive my results,  is to
> create intermediate temporary pipe inside kernel and splice from
> non-pipe fd to pipe, and than from pipe to second non-pipe fd.

Have a look at the "sendfile emulation" in fs/splice.c. Maybe you can
generalize that in some way or the other.

Enrik

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