netdev - Re: maximum buffer size for splice(2) tcp->pipe?

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Date:	Thu, 15 Jan 2009 12:47:22 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	David Miller <davem@...emloft.net>
CC:	akpm@...ux-foundation.org, Volker.Lendecke@...Net.DE,
	linux-kernel@...r.kernel.org, sfrench@...ibm.com,
	jens.axboe@...cle.com, netdev@...r.kernel.org
Subject: Re: maximum buffer size for splice(2) tcp->pipe?

David Miller a écrit :
> From: Eric Dumazet <dada1@...mosbay.com>
> Date: Wed, 14 Jan 2009 00:38:32 +0100
> 
>> [PATCH] net: splice() from tcp to socket should take into account O_NONBLOCK
>>
>> Instead of using SPLICE_F_NONBLOCK to select a non blocking mode both on
>> source tcp socket and pipe destination, we use the underlying file flag (O_NONBLOCK)
>> for selecting a non blocking socket.
>>
>> Signed-off-by: Eric Dumazet <dada1@...mosbay.com>
> 
> This needs at least some more thought.
> 
> It seems, for one thing, that this change will interfere with the
> intentions of the code in splice_dirt_to_actor which goes:
> 
> 	/*
> 	 * Don't block on output, we have to drain the direct pipe.
> 	 */
> 	sd->flags &= ~SPLICE_F_NONBLOCK;

Reading splice_direct_to_actor() I see nothing wrong with the patch

(Patch is about splice from socket to pipe, while the sd->flags you mention
in splice_direct_to_actor() only applies to the splice from internal pipe to
something else, as splice_direct_to_actor() allocates an internal pipe to perform
its work.

Also, the meaning of SPLICE_F_NONBLOCK, as explained in include/linux/splice.h is :

#define SPLICE_F_NONBLOCK (0x02) /* don't block on the pipe splicing (but */
	/* we may still block on the fd we splice */
	/* from/to, of course */

If the comment is still correct, SPLICE_F_NONBLOCK only applies to the pipe implied in
splice() syscall.

For the other file, either its :
- A regular file : nonblocking mode is not available, like a normal read()/write() syscall

- A socket : We should be able to specify if its blocking or not, independantly from
             the SPLICE_F_NONBLOCK flag that only applies to the pipe. Normal way
             is using ioctl(FIONBIO) or other fcntl() call to change file->f_flags O_NONBLOCK


In order to be able to efficiently use splice() from a socket to a file, we need
to do a loop of :

{
splice(from blocking tcp socket to non blocking pipe, SPLICE_F_NONBLOCK); /* nonblocking pipe or risk deadlock */
splice(from pipe to file)
}


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