On Thu, Jun 4, 2015 at 1:56 PM, Martin KaFai Lau <ka...@fb.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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