netdev - Re: [REGRESSION] Warning in tcp_fastretrans_alert() of net/ipv4/tcp

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Date:   Mon, 6 Nov 2017 14:27:30 -0800
From:   Yuchung Cheng <ycheng@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Roman Gushchin <guro@...com>, netdev <netdev@...r.kernel.org>,
        Neal Cardwell <ncardwell@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Lawrence Brakmo <brakmo@...com>
Subject: Re: [REGRESSION] Warning in tcp_fastretrans_alert() of net/ipv4/tcp_input.c

On Fri, Oct 27, 2017 at 1:38 PM, Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Wed, Oct 25, 2017 at 10:37 PM, Yuchung Cheng <ycheng@...gle.com> wrote:
> > On Wed, Oct 25, 2017 at 7:07 PM, Alexei Starovoitov
> > <alexei.starovoitov@...il.com> wrote:
> >>
> >> On Thu, Sep 28, 2017 at 04:36:58PM -0700, Yuchung Cheng wrote:
> >> > On Thu, Sep 28, 2017 at 1:14 AM, Oleksandr Natalenko
> >> > <oleksandr@...alenko.name> wrote:
> >> > > Hi.
> >> > >
> >> > > Won't tell about panic in tcp_sacktag_walk() since I cannot trigger it
> >> > > intentionally, but setting net.ipv4.tcp_retrans_collapse to 0 *does not* fix
> >> > > warning in tcp_fastretrans_alert() for me.
> >> >
> >> > Hi Oleksandr: no retrans_collapse should not matter for that warning
> >> > in tcp_fstretrans_alert(). the warning as I explained earlier is
hi guys can you try if the warning goes away w/ this quick fix?


diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 0ada8bfc2ebd..072aab2a8226 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2626,7 +2626,7 @@ void tcp_simple_retransmit(struct sock *sk)

        tcp_clear_retrans_hints_partial(tp);

-       if (prior_lost == tp->lost_out)
+       if (!tp->lost_out)
                return;

        if (tcp_is_reno(tp))



> >> > likely false. Neal and I are more concerned the panic in
> >> > tcp_sacktag_walk. This is just a blind shot but thx for retrying.
> >> >
> >> > We can submit a one-liner to remove the fast retrans warning but want
> >> > to nail the bigger issue first.
> >>
> >> we're still seeing the warnings followed by crashes and it's very concerning.
> >> We hoped that most recent Neal's patches from Sep 18 around this area may
> >> magically fix the issue, but no. The panics are still there.
> >> It's confirmed that net.ipv4.tcp_retrans_collapse=0 does not help
> >> whereas net.ipv4.tcp_recovery=0 works, but obviously undesirable.
> >> We're out of ideas on how to debug this.
> > Can you try Eric's latest SACK rb-tree patches?
> > https://patchwork.ozlabs.org/cover/822218/
> >
> > Roman's SNMP data suggests MTU probing is enabled. Another blind shot
> > is to disable it.
>
>
> Or alternatively try this fix :
>
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 1151870018e345592853b035a0902121c41e268d..6a849c7028f06f31b36a906be37995b28b579a40
> 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -2062,6 +2062,8 @@ static int tcp_mtu_probe(struct sock *sk)
>         nskb->ip_summed = skb->ip_summed;
>
>         tcp_insert_write_queue_before(nskb, skb, sk);
> +       if (skb == tp->highest_sack)
> +               tp->highest_sack = nskb;
>
>         len = 0;
>         tcp_for_write_queue_from_safe(skb, next, sk) {