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From: Eelco Chaudron <echaudro@redhat.com>
To: Ilya Maximets <i.maximets@ovn.org>
Cc: Aaron Conole <aconole@redhat.com>, dev@openvswitch.org,
 netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
 Eric Dumazet <edumazet@google.com>, Jakub Kicinski <kuba@kernel.org>,
 Paolo Abeni <pabeni@redhat.com>, "David S. Miller" <davem@davemloft.net>
Subject: Re: [ovs-dev] [PATCH net] openvswitch: Set the skbuff pkt_type for
 proper pmtud support.
Date: Mon, 25 Mar 2024 13:57:22 +0100
Message-ID: <4C04D4FF-0ADF-45DC-B253-2CD5C997DA1B@redhat.com>
In-Reply-To: <4066cc6a-24a8-4d05-b180-99222fe792fa@ovn.org>
References: <20240322190603.251831-1-aconole@redhat.com>
 <7AFF5D6D-568C-449B-83CF-9436DE97CA91@redhat.com>
 <f7t5xxawlen.fsf@redhat.com> <4066cc6a-24a8-4d05-b180-99222fe792fa@ovn.org>
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On 25 Mar 2024, at 13:37, Ilya Maximets wrote:

> On 3/25/24 13:22, Aaron Conole wrote:
>> Eelco Chaudron <echaudro@redhat.com> writes:
>>
>>> On 22 Mar 2024, at 20:06, Aaron Conole wrote:
>>>
>>>> Open vSwitch is originally intended to switch at layer 2, only deali=
ng with
>>>> Ethernet frames.  With the introduction of l3 tunnels support, it cr=
ossed
>>>> into the realm of needing to care a bit about some routing details w=
hen
>>>> making forwarding decisions.  If an oversized packet would need to b=
e
>>>> fragmented during this forwarding decision, there is a chance for pm=
tu
>>>> to get involved and generate a routing exception.  This is gated by =
the
>>>> skbuff->pkt_type field.
>>>>
>>>> When a flow is already loaded into the openvswitch module this field=
 is
>>>> set up and transitioned properly as a packet moves from one port to
>>>> another.  In the case that a packet execute is invoked after a flow =
is
>>>> newly installed this field is not properly initialized.  This causes=
 the
>>>> pmtud mechanism to omit sending the required exception messages acro=
ss
>>>> the tunnel boundary and a second attempt needs to be made to make su=
re
>>>> that the routing exception is properly setup.  To fix this, we set t=
he
>>>> outgoing packet's pkt_type to PACKET_OUTGOING, since it can only get=

>>>> to the openvswitch module via a port device or packet command.
>>>
>>> Is this not a problem when the packet comes from the bridge port in t=
he kernel?
>>
>> It very well may be an issue there as well, but the recommendation is =
to
>> operate with the bridge port down as far as I know, so I don't know if=

>> this issue has been observed happening from the bridge port.
>
> FWIW, bridge ports are typically used as an entry point for tunneled
> traffic so it can egress from a physical port attached to OVS.  It mean=
s
> they are pretty much always UP in most common setups like OpenStack or
> ovn-kubernetes and handle a decent amount of traffic.  They are also us=
ed
> to direct some other types of traffic to the host kernel.

+1 here, I=E2=80=99m talking about the same port. I think we only advise =
having this down for userspace bridges, but not in the case the bridge is=
 the tunnel endpoint.

> Unless I misunderstood which ports we're talking about here.
>
> Best regards, Ilya Maximets.