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Date:	Mon, 15 Oct 2007 01:47:28 -0700 (PDT)
From:	David Miller <>
Subject: Re: [NET]: Avoid copying TCP packets unnecessarily

From: Herbert Xu <>
Date: Mon, 15 Oct 2007 13:54:57 +0800

> On Mon, Oct 15, 2007 at 12:57:23PM +0800, Herbert Xu wrote:
> > 
> > However, the fact that you're triggering this code at all tells
> > us that there is another bug further up the chain that's causing
> > us to generate partial checksums even though the NIC doesn't
> > support it.  Can you please give me the output of
> Nevermind, tcp_fragment can generate such packets out of
> convenience.
> Here's a little optimisation to avoid copying the packet in
> this case.
> [NET]: Avoid copying TCP packets unnecessarily
> TCP packets all have writable heads, that is, even though it's cloned, it is
> writable up to the end of the TCP header.  This patch makes skb_checksum_help
> aware of this fact by using skb_clone_writable and avoiding a copy for TCP.
> I've also modified the BUG_ON tests to be unsigned.  The only case where this
> makes a difference is if csum_start points to a location before skb->data.
> Since skb->data should always include the header where the checksum field
> is (and all currently callers adhere to that), this change is safe and may
> uncover bugs later.
> Signed-off-by: Herbert Xu <>

Applied, thanks Herbert!
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