netdev - Re: pskb_copy() in tcp_transmit

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Date:	Tue, 13 Sep 2011 14:02:17 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	christoph.paasch@...ouvain.be
Cc:	Gaofeng <gaofeng@...fujitsu.com>, netdev@...r.kernel.org
Subject: Re: pskb_copy() in tcp_transmit_skb()

Le 13 septembre 2011 12:55, Christoph Paasch
<christoph.paasch@...ouvain.be> a écrit :
>
> Hi,
>
> On Tuesday 13 September 2011 wrote Gaofeng:
> > > I'm trying to understand the reason for the possible call to pskb_copy()
> > > in tcp_transmit_skb().
> > > I don't find, where we may have a cloned skb entering tcp_transmit_skb().
> > >
> > > The original pskb_copy() came from tcp_retransmit_skb() (commit
> > > dfb4b9dceb35c567a595ae5e9d035cfda044a103).
> > > But from that point, the git-history does not show where the pskb_copy()
> > > is coming from.
> > >
> > >
> > > Does somebody has an idea, how a cloned skb can be passed to
> > > tcp_transmit_skb() and thus trigger this call to pskb_copy().
> >
> > maybe nat,nat can change the tcp packet header
>
> although NAT may change the TCP header, it is not doing an skb_clone() - (at
> least I don't find it).
> Thus, we will not call pskb_copy() in tcp_transmit_skb() due to NAT.
>

I suggest you read dev_queue_xmit_nit() : Every xmit packet can be
cloned right here.

By the time tcp_retransmit_skb() is called, cloned skb might still be
in a AF_PACKET queue (or even a device TX queue)
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