netdev - Re: Recurring trace from tcp

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Date:	Thu, 4 Jun 2015 14:29:59 -0700
From:	Yuchung Cheng <ycheng@...gle.com>
To:	Martin KaFai Lau <kafai@...com>
Cc:	Grant Zhang <gzhang@...tly.com>, netdev <netdev@...r.kernel.org>,
	Neal Cardwell <ncardwell@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Kernel Team <kernel-team@...com>
Subject: Re: Recurring trace from tcp_fragment()

On Thu, Jun 4, 2015 at 1:56 PM, Martin KaFai Lau <kafai@...com> wrote:
>
> On Thu, Jun 04, 2015 at 01:10:26PM -0700, Grant Zhang wrote:
> > Hi Martin,
> >
> > Thank you! My net.ipv4.tcp_mtu_probing is 1. After turning it off,
> > the WARN_ON stack is gone.
> Thanks for confirming it.
>
> > Could you elaborate a bit on why this setting relates to the WARN_ON
> > trace?
> The WARN_ON is complaining about tcp_fragment() is trying to slice
> a skb which has a too-short skb->len.
>
> When doing mtu probing, it may slice the skb.  In some cases (which
> I also failed to reproduce in packetdrill), it does not
> update some related skb values and then confuse tcp_fragment()
> later on.
>
> > And what are the pros/cons for disabling mtu_probing?
> It depends on your traffic, I guess.  However, turning it off is not the
> right fix.
There might be two bugs. We saw the warning with mtu_probing=0. This
would explain why Neal's fix did not work.


>
> FYI, here is the change I am trying.
>
> Thanks,
> --Martin
>
> diff --git i/net/ipv4/tcp_output.c w/net/ipv4/tcp_output.c
> index acec745..e767e53 100644
> --- i/net/ipv4/tcp_output.c
> +++ w/net/ipv4/tcp_output.c
> @@ -1920,6 +1920,8 @@ static int tcp_mtu_probe(struct sock *sk)
>                                                    ~(TCPHDR_FIN|TCPHDR_PSH);
>                         if (!skb_shinfo(skb)->nr_frags) {
>                                 skb_pull(skb, copy);
> +                               if (tcp_skb_pcount(skb) > 1)
> +                                       tcp_set_skb_tso_segs(sk, skb, mss_now);
>                                 if (skb->ip_summed != CHECKSUM_PARTIAL)
>                                         skb->csum = csum_partial(skb->data,
>                                                                  skb->len, 0);
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