Date:	Mon, 01 Dec 2014 14:17:28 +0100
From:	Wolfgang Walter <linux@...m.de>
To:	Thomas Jarosch <thomas.jarosch@...ra2net.com>
Cc:	netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>
Subject: Re: [bisected] xfrm: TCP connection initiating PMTU discovery stalls on v3.12+

Am Samstag, 29. November 2014, 12:44:07 schrieb Thomas Jarosch:
> Hello,
> 
> we're in the process of updating production level machines
> from kernel 3.4.101 to kernel 3.14.25. On one mail server
> we noticed that emails destined for an IPSec tunnel sometimes
> get stuck in the mail queue with TCP timeouts.
> 
> To make a long story short: When the VPN connection is initially
> set up or re-newed, the path MTU for the xfrm tunnel is undetermined.
> 
> As soon as a TCP client starts to send large packets,
> it triggers path MTU detection. Some middlebox on the
> way to the final server has a lower MTU and sends back
> an "ICMP fragmentation needed" packet as normal.
> 
> With the old kernel, the packet size for the TCP connection inside
> the xfrm tunnel gets adjusted and all is fine. With kernel v3.12+,
> the connection stalls completely. Same thing with kernel v3.18-rc6.

We see something similar with real nic (RTL8139). In our case only the first 
tcp-connection which triggers PMTU stalls. Later tcp-connections then work 
fine.

I will revert that patch and see if that fixes the problem.

> 
> We wrote a small tool to mimic postfix's TCP behavior (see attached file).
> In the end it's a normal TCP client sending large packets.
> The server side is just "socat - tcp4-listen:667".
> 
> If you run "socket_client" a second time, the path MTU
> for the xfrm tunnel is already known and packets flow normal, too.
> 
> 
> The "evil" commit in question is this one:
> ---------------------------------------------------------------------
> commit 8f26fb1c1ed81c33f5d87c5936f4d9d1b4118918
> Author: Eric Dumazet <edumazet@...gle.com>
> Date:   Tue Oct 15 12:24:54 2013 -0700
> 
>     tcp: remove the sk_can_gso() check from tcp_set_skb_tso_segs()
> 
>     sk_can_gso() should only be used as a hint in tcp_sendmsg() to build GSO
> packets in the first place. (As a performance hint)
> 
>     Once we have GSO packets in write queue, we can not decide they are no
>     longer GSO only because flow now uses a route which doesn't handle
>     TSO/GSO.
> 
>     Core networking stack handles the case very well for us, all we need
>     is keeping track of packet counts in MSS terms, regardless of
>     segmentation done later (in GSO or hardware)
> 
>     Right now, if  tcp_fragment() splits a GSO packet in two parts,
>     @left and @right, and route changed through a non GSO device,
>     both @left and @right have pcount set to 1, which is wrong,
>     and leads to incorrect packet_count tracking.
> 
>     This problem was added in commit d5ac99a648 ("[TCP]: skb pcount with MTU
> discovery")
> 
>     Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>     Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
>     Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
>     Reported-by: Maciej Żenczykowski <maze@...gle.com>
>     Signed-off-by: David S. Miller <davem@...emloft.net>
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 8fad1c1..d46f214 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -989,8 +989,7 @@ static void tcp_set_skb_tso_segs(const struct sock *sk,
> struct sk_buff *skb, /* Make sure we own this skb before messing
> gso_size/gso_segs */ WARN_ON_ONCE(skb_cloned(skb));
> 
> -       if (skb->len <= mss_now || !sk_can_gso(sk) ||
> -           skb->ip_summed == CHECKSUM_NONE) {
> +       if (skb->len <= mss_now || skb->ip_summed == CHECKSUM_NONE) {
>                 /* Avoid the costly divide in the normal
>                  * non-TSO case.
>                  */
> ---------------------------------------------------------------------
> 
> When I revert it, even kernel v3.18-rc6 starts working.
> But I doubt this is the root problem, may be just hiding another issue.
> 
> --- Sample output of socket_client using vanilla v3.12 kernel ---
> [1417258063 SEND result: 4096, strerror: Success]
> tcp max seg: res: 0, max_seg: 1370
> [1417258063 SEND result: 4096, strerror: Success]
> tcp max seg: res: 0, max_seg: 1370
> [1417258063 SEND result: 4096, strerror: Success]
> tcp max seg: res: 0, max_seg: 1370
> [1417258063 SEND result: 4096, strerror: Success]
> tcp max seg: res: 0, max_seg: 1370
> [1417258063 SEND result: 4096, strerror: Success]
> tcp max seg: res: 0, max_seg: 1338
> [1417258063 SEND result: 4096, strerror: Success]
> tcp max seg: res: 0, max_seg: 1338
> *STUCK*
> --------------------------------------------------------
> 
> The "machine" is running on KVM and using "virtio_net" as NIC driver.
> I've played with the ethtool offload settings:
> 
> *** eth1 defaults ***
> Offload parameters for eth1:
> rx-checksumming: on
> tx-checksumming: on
> scatter-gather: on
> tcp-segmentation-offload: on
> udp-fragmentation-offload: on
> generic-segmentation-offload: on
> generic-receive-offload: on
> large-receive-offload: off
> 
> *** eth1 working (no stalls) using vanilla kernel ***
> Offload parameters for eth1:
> rx-checksumming: on
> tx-checksumming: off  <-- the magic switch
> scatter-gather: off
> tcp-segmentation-offload: off
> udp-fragmentation-offload: off
> generic-segmentation-offload: off
> generic-receive-offload: off
> large-receive-offload: off
> 
> When I turn "tx-checksumming" back on, it fails again.
> Though that is probably also just a side effect.
> 
> I can provide tcpdumps if needed but they are no real help
> since you can just see the kernel stops sending TCP packets.
> (and the outgoing TCP packets are encrypted in ESP packets)
> 
> 
> Any vague idea what might be the root cause?
> 
> I also tried reverting commit 4d53eff48b5f03ce67f4f301d6acca1d2145cb7a
> ("xfrm: Don't queue retransmitted packets if the original is still on the
> host") but that didn't change the situation. In fact it wasn't even
> triggered.
> 
> Please CC: comments. Thanks.
> 
> Best regards,
> Thomas

Regards,
-- 
Wolfgang Walter
Studentenwerk München
Anstalt des öffentlichen Rechts
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