netdev - Re: Recurring trace from tcp

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Message-ID: <20150604205629.GB2951343@devbig242.prn2.facebook.com>
Date:	Thu, 4 Jun 2015 13:56:29 -0700
From:	Martin KaFai Lau <kafai@...com>
To:	Grant Zhang <gzhang@...tly.com>
CC:	netdev <netdev@...r.kernel.org>,
	Neal Cardwell <ncardwell@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Kernel Team <kernel-team@...com>
Subject: Re: Recurring trace from tcp_fragment()

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.

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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