netdev - Re: Data corruption issue with splice() on 2.6.27.10

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Message-ID: <20090107125756.GA32336@ioremap.net>
Date:	Wed, 7 Jan 2009 15:57:56 +0300
From:	Evgeniy Polyakov <zbr@...emap.net>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Willy Tarreau <w@....eu>, Jens Axboe <jens.axboe@...cle.com>,
	Jarek Poplawski <jarkao2@...il.com>,
	Changli Gao <xiaosuo@...il.com>, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: Data corruption issue with splice() on 2.6.27.10

On Wed, Jan 07, 2009 at 11:53:56PM +1100, Herbert Xu (herbert@...dor.apana.org.au) wrote:
> On Wed, Jan 07, 2009 at 01:52:01PM +0100, Willy Tarreau wrote:
> > 
> > Evgeniy, I'd like to understand something related to our apparent lack of
> > knowledge of when the data is effectively transmitted. If we're focusing
> > on the send part, I can't understand why I never reproduce the corruption
> > when the data source is a file or loopback, but I only see it when the source
> > is an ethernet interface. How is it possible that a problem affecting only
> 
> It doesn't happen with a file because in that case you don't
> start with an skb so there is no skb->head.  It probably doesn't
> happen with loopback because loopback does GSO so again skb->head
> does not exist (so to speak).

Yup, basically splice's transmit pipe buffer contains page references,
where the first one is actually not a real page but skb, while in the
case of sendfile() and/or splice() from the file first page is a real
page of the appropriate file.

-- 
	Evgeniy Polyakov
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