[3.16.y-ckt stable] Patch "tcp: Do not apply TSO segment limit to non-TSO packets" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    tcp: Do not apply TSO segment limit to non-TSO packets

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt5.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From c17a800343f38a22bb25ba26f3360c73681e68e8 Mon Sep 17 00:00:00 2001
From: Herbert Xu <herbert at gondor.apana.org.au>
Date: Thu, 1 Jan 2015 00:39:23 +1100
Subject: tcp: Do not apply TSO segment limit to non-TSO packets

commit 843925f33fcc293d80acf2c5c8a78adf3344d49b upstream.

Thomas Jarosch reported IPsec TCP stalls when a PMTU event occurs.

In fact the problem was completely unrelated to IPsec.  The bug is
also reproducible if you just disable TSO/GSO.

The problem is that when the MSS goes down, existing queued packet
on the TX queue that have not been transmitted yet all look like
TSO packets and get treated as such.

This then triggers a bug where tcp_mss_split_point tells us to
generate a zero-sized packet on the TX queue.  Once that happens
we're screwed because the zero-sized packet can never be removed
by ACKs.

Fixes: 1485348d242 ("tcp: Apply device TSO segment limit earlier")
Reported-by: Thomas Jarosch <thomas.jarosch at intra2net.com>
Signed-off-by: Herbert Xu <herbert at gondor.apana.org.au>

Cheers,
Signed-off-by: David S. Miller <davem at davemloft.net>
[ luis: backported to 3.16: used davem's backport to 3.18 ]
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 net/ipv4/tcp_output.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index a8794367cd20..64fb6e84127e 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1935,7 +1935,7 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
 		if (unlikely(!tcp_snd_wnd_test(tp, skb, mss_now)))
 			break;

-		if (tso_segs == 1) {
+		if (tso_segs == 1 || !sk->sk_gso_max_segs) {
 			if (unlikely(!tcp_nagle_test(tp, skb, mss_now,
 						     (tcp_skb_is_last(sk, skb) ?
 						      nonagle : TCP_NAGLE_PUSH))))
@@ -1971,7 +1971,7 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
 		}

 		limit = mss_now;
-		if (tso_segs > 1 && !tcp_urg_mode(tp))
+		if (tso_segs > 1 && sk->sk_gso_max_segs && !tcp_urg_mode(tp))
 			limit = tcp_mss_split_point(sk, skb, mss_now,
 						    min_t(unsigned int,
 							  cwnd_quota,
--
2.1.4