linux-kernel - Re: [ovs-dev] [PATCH net] openvswitch: Set the skbuff pkt

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Message-ID: <4C04D4FF-0ADF-45DC-B253-2CD5C997DA1B@redhat.com>
Date: Mon, 25 Mar 2024 13:57:22 +0100
From: Eelco Chaudron <echaudro@...hat.com>
To: Ilya Maximets <i.maximets@....org>
Cc: Aaron Conole <aconole@...hat.com>, dev@...nvswitch.org,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, "David S. Miller" <davem@...emloft.net>
Subject: Re: [ovs-dev] [PATCH net] openvswitch: Set the skbuff pkt_type for
 proper pmtud support.



On 25 Mar 2024, at 13:37, Ilya Maximets wrote:

> On 3/25/24 13:22, Aaron Conole wrote:
>> Eelco Chaudron <echaudro@...hat.com> writes:
>>
>>> On 22 Mar 2024, at 20:06, Aaron Conole wrote:
>>>
>>>> Open vSwitch is originally intended to switch at layer 2, only dealing with
>>>> Ethernet frames.  With the introduction of l3 tunnels support, it crossed
>>>> into the realm of needing to care a bit about some routing details when
>>>> making forwarding decisions.  If an oversized packet would need to be
>>>> fragmented during this forwarding decision, there is a chance for pmtu
>>>> 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 across
>>>> the tunnel boundary and a second attempt needs to be made to make sure
>>>> that the routing exception is properly setup.  To fix this, we set the
>>>> 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 the 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 means
> 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 used
> to direct some other types of traffic to the host kernel.

+1 here, I’m 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.