netdev - Re: [REGRESSION, bisect]cxgb4 port failure with TSO traffic after commit 10d3be569243def8("tcp-tso: do not split TSO packets at retransmit time")

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Date:	Mon, 27 Jun 2016 14:46:56 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Arjun V <arjun@...lsio.com>
Cc:	Eric Dumazet <edumazet@...gle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Hariprasad S <hariprasad@...lsio.com>,
	Casey Leedom <leedom@...lsio.com>,
	Kumar A S <kumaras@...lsio.com>,
	Santosh Rastapur <santosh@...lsio.com>,
	Nirranjan Kirubaharan <nirranjan@...lsio.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"ycheng@...gle.com" <ycheng@...gle.com>
Subject: Re: [REGRESSION, bisect]cxgb4 port failure with TSO traffic after
 commit 10d3be569243def8("tcp-tso: do not split TSO packets at retransmit
 time")

On Mon, 2016-06-27 at 12:50 +0200, Eric Dumazet wrote:
> On Mon, 2016-06-27 at 15:24 +0530, Arjun V wrote:
> 
> > Applied your patch.
> > The above debug print is not getting invoked, when skb->len is greater than 65536.
> 
> Interesting.
> 
> It looks like we might have a bug in tcp_shift_skb_data() not respecting
> sk->sk_gso_max_size at all ...

Note that this also means TCP receiver misbehaves and force us to SACK
reneging.

Patch would be :

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 8bd9911fdd16..3587efe22864 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2784,6 +2784,10 @@ void tcp_xmit_retransmit_queue(struct sock *sk)
 		segs = tp->snd_cwnd - tcp_packets_in_flight(tp);
 		if (segs <= 0)
 			return;
+		/* In case tcp_shift_skb_data() have aggregated large skbs,
+		 * we need to make sure not sending too big TSO packets.
+		 */
+		segs = min_t(int, segs, tp->gso_segs);
 
 		if (fwd_rexmitting) {
 begin_fwd: