[ovs-dev] Odd gre_sys MTU setting (2.6.x vs 2.8.x)

[Christian Ehrhardt]

On Mon, Jan 15, 2018 at 5:43 PM, Eric Garver <e at erig.me> wrote:
> On Mon, Jan 15, 2018 at 11:32:32AM -0500, Eric Garver wrote:
>> On Mon, Jan 15, 2018 at 03:17:28PM +0000, James Page wrote:
>> > Hi Eric
>> >
>> > On Mon, 15 Jan 2018 at 16:54 Eric Garver <e at erig.me> wrote:
>> >
>> > > On Sat, Jan 13, 2018 at 03:57:08PM +0000, James Page wrote:
>> > > > Hi Ben s
>> > > >
>> > > > On Sat, 13 Jan 2018 at 14:55 James Page <james.page at ubuntu.com> wrote:
>> > > >
>> > > > > OK, I sent a patch:
>> > > > >>         https://patchwork.ozlabs.org/patch/860192/
>> > > > >
>> > > > >
>> > > > > Thanks Ben - I'll pull this patch into the Ubuntu packages and test
>> > > this
>> > > > > weekend.
>> > > > >
>> > > >
>> > > >  I pulled your patch in ontop of our current 2.8.1 packages and
>> > > re-tested;
>> > > > I'm not seeing a difference in behaviour with the patch in place. I
>> > > removed
>> > > > the vport_gre and ip_gre kernel modules to force a re-creation of the
>> > > > device on restart of OVS as well as trying a reboot of the test machine:
>> > > >
>> > > > 7: gre0 at NONE: <NOARP> mtu 1476 qdisc noop state DOWN mode DEFAULT group
>> > > > default qlen 1
>> > > >     link/gre 0.0.0.0 brd 0.0.0.0
>> > > > 8: gretap0 at NONE: <BROADCAST,MULTICAST> mtu 1462 qdisc noop state DOWN
>> > > mode
>> > > > DEFAULT group default qlen 1000
>> > > >     link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
>> > > > 9: gre_sys at NONE: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1472 qdisc
>> > > > pfifo_fast master ovs-system state UNKNOWN mode DEFAULT group default
>> > > qlen
>> > > > 1000
>> > > >     link/ether 52:ad:6d:89:bb:54 brd ff:ff:ff:ff:ff:ff
>> > >
>> > > Are there any errors is dmesg? It's possible setlink won't accept
>> > > UINT16_MAX. grep for "Invalid MTU".
>> > >
>> >
>> > Not under 4.4 but I do under 4.13:
>> >
>> > [    6.794079] gre: GRE over IPv4 demultiplexor driver
>> > [    6.797621] ip_gre: GRE over IPv4 tunneling driver
>> > [    6.798326] gre_sys: Invalid MTU 65535 requested, hw max 1500
>> >
>> > Looks like the kernel is limiting to 1500.
>>
>> I would expect setting with ip-link to fail as well.
>> What does the below show?:
>>
>>   $ ip link set dev gre_sys mtu 65535
>
> Ugh. This is a separate kernel bug fixed by this commit:
>
>     commit cfddd4c33c254954927942599d299b3865743146
>     Author: Xin Long <lucien.xin at gmail.com>
>     Date:   Mon Dec 18 14:24:35 2017 +0800
>
>     ip_gre: remove the incorrect mtu limit for ipgre tap

Thanks Eric, that matches my findings, glad that there seems to be an
accepted fix already.
But it is fairly recent and only in since 4.15-rc8 levels afaik.

But OTOH its description at [1] reads pretty much like my notes so far.

@James - do you think you could test a super-recent mainline kernel
build from [2] in regard to this issue?

[1]: https://github.com/torvalds/linux/commit/cfddd4c33c254954927942599d299b3865743146
[2]: http://kernel.ubuntu.com/~kernel-ppa/mainline/v4.15-rc8/