netdev - Re: Bug in computing data_len in tcp

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Message-ID: <1322778621.2750.48.camel@edumazet-laptop>
Date:	Thu, 01 Dec 2011 23:30:21 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Vijay Subramanian <subramanian.vijay@...il.com>
Cc:	Tom Herbert <therbert@...gle.com>,
	Linux Netdev List <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: Bug in computing data_len in tcp_sendmsg?

Le jeudi 01 décembre 2011 à 14:18 -0800, Vijay Subramanian a écrit :
> >
> > Or the "bug" was to assume that skb was headless.
> > It was true until recently.
> >
> > We recently added commit f07d960df33c5aef
> > (tcp: avoid frag allocation for small frames)
> >
> > to avoid page allocation for small frames.
> >
> > So now, skb can contain in head part of tcp data.
> >
> >
> 
> Hi,
> I am looking at tcp_mtu_probe() and was wondering if this commit also
> impacts this function. Once the data are copied from skbs in the write
> queue to the probe skb, copied data are cleared from the original skbs
> in the write queue.
> 
> It looks like the code assumes that the original skb will have data
> either in linear part or in paged part.  The call to
> __pskb_trim_head(skb, copy) for example does not clear linear part.
> 
> Can someone more familiar with the code take a look? Apologies if I
> have read this wrong.
> 

tcp_mtu_probe() builds a linear skb, and populate it using
skb_copy_bits() [ this is frag aware, and aware of payload in header as
well ]

I see no problem in it.



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