linux-kernel - Re: [PATCH net-next v5 2/2] L2TP:Adjust intf MTU, add underlay L3, L2 hdrs.

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Date:   Tue, 11 Apr 2017 19:05:56 +0200
From:   Guillaume Nault <g.nault@...halink.fr>
To:     R Parameswaran <parameswaran.r7@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        James Chapman <jchapman@...alix.com>, kleptog@...na.org,
        Nachi Prachanda <nprachan@...cade.com>,
        Robert Shearman <rshearma@...cade.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Sven-Thorsten Dietrich <sdietric@...cade.com>,
        Chas Williams III <ciwillia@...cade.com>,
        Luca Boccassi <lboccass@...cade.com>,
        Derek Fawcus <dfawcus@...cade.com>,
        Bill Hong <bhong@...cade.com>,
        Jan Blunck <jblunck@...cade.com>,
        David Miller <davem@...emloft.net>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next v5 2/2] L2TP:Adjust intf MTU, add underlay L3,
 L2 hdrs.

On Tue, Apr 11, 2017 at 09:39:58AM -0700, R Parameswaran wrote:
> Hi Guillaume,
> 
> On Tue, Apr 11, 2017 at 3:40 AM, Guillaume Nault <g.nault@...halink.fr> wrote:
> > On Wed, Apr 05, 2017 at 05:00:07PM -0700, R. Parameswaran wrote:
> >>
> >> Change-set here uses the new kernel function, kernel_sock_ip_overhead(),
> >> to factor the outer IP overhead on the L2TP tunnel socket (including
> >> IP Options, if any) when calculating the default MTU for an Ethernet
> >> pseudowire, along with consideration of the inner Ethernet header.
> >>
> > I get the following warning with CONFIG_LOCKDEP when creating a new
> > session:
> > # ip l2tp add tunnel local 10.1.8.64 remote 10.1.8.32 udp_sport 1701 udp_dport 1701 tunnel_id 1 peer_tunnel_id 1
> > # ip l2tp add session tunnel_id 1 session_id 1 peer_session_id 1
> > ...
> 
> Thanks for reporting this - I'll try and put up a patch soon,
> hopefully the patch can stay in while I add this. One Q - how many CPU
> cores do you have?
This is a virtual machine with 4 vcores, but that shouldn't matter.

> Can you give me some idea of how many tunnels and
> sessions when you saw this?
> 
Creating one session is enough. I simply used the following command:
# ip l2tp add tunnel local 10.1.8.64 remote 10.1.8.32 udp_sport 1701 udp_dport 1701 tunnel_id 1 peer_tunnel_id 1
# ip l2tp add session tunnel_id 1 session_id 1 peer_session_id 1

> I did not see this warning in my testing, possibly because
> CONFIG_LOCKDEP_SUPPORT is turned off on the product build? Will
> re-test with this turned on.
> 
Yes, enabling lockdep should let you reproduce the problem.

The issue goes away if the tunnel's socket is locked while calling
kernel_sock_ip_overhead():
+	lock_sock(tunnel->sock);
	kernel_sock_ip_overhead(tunnel->sock);
+	release_sock(tunnel->sock);