Message-ID: <4164872.1744747795@famine>
Date: Tue, 15 Apr 2025 13:09:55 -0700
From: Jay Vosburgh <jv@...sburgh.net>
To: David Wilder <wilder@...ibm.com>
cc: Ilya Maximets <i.maximets@....org>,
    "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
    "pradeeps@...ux.vnet.ibm.com" <pradeeps@...ux.vnet.ibm.com>,
    Pradeep Satyanarayana <pradeep@...ibm.com>,
    Adrian Moreno Zapata <amorenoz@...hat.com>,
    Hangbin Liu <haliu@...hat.com>
Subject: Re: [PATCH net-next v1 1/1] bonding: Adding limmited support for ARP
 monitoring with ovs.

David Wilder <wilder@...ibm.com> wrote:

>
>
>>>> Adding limited support for the ARP Monitoring feature when ovs is
>>>> configured above the bond. When no vlan tags are used in the configuration
>>>> or when the tag is added between the bond interface and the ovs bridge arp
>>>> monitoring will function correctly. The use of tags between the ovs bridge
>>>> and the routed interface are not supported.
>>>
>>>       Looking at the patch, it isn't really "adding support," but
>>> rather is disabling the "is this IP address configured above the bond"
>>> checks if the bond is a member of an OVS bridge.  It also seems like it
>>> would permit any ARP IP target, as long as the address is configured
>>> somewhere on the system.
>>>
>>>       Stated another way, the route lookup in bond_arp_send_all() for
>>> the target IP address must return a device, but the logic to match that
>>> device to the interface stack above the bond will always succeed if the
>>> bond is a member of any OVS bridge.
>>>
>>>       For example, given:
>>>
>>> [ eth0, eth1 ] -> bond0 -> ovs-br -> ovs-port IP=10.0.0.1
>>> eth2 IP=20.0.0.2
>>>
>>>       Configuring arp_ip_target=20.0.0.2 on bond0 would apparently
>>> succeed after this patch is applied, and the bond would send ARPs for
>>> 20.0.0.2.
>>>
>>>> For example:
>>>> 1) bond0 -> ovs-br -> ovs-port (x.x.x.x) is supported
>>>> 2) bond0 -> bond0.100 -> ovs-br -> ovs-port (x.x.x.x) is supported.
>>>> 3) bond0 -> ovs-br -> ovs-port -> ovs-port.100 (x.x.x.x) is not supported.
>>>>
>>>> Configurations #1 and #2 were tested and verified to function corectly.
>>>> In the second configuration the correct vlan tags were seen in the arp.
>>>
>>>       Assuming that I'm understanding the behavior correctly, I'm not
>>> sure that "if OVS then do whatever" is the right way to go, particularly
>>> since this would still exhibit mysterious failures if a VLAN is
>>> configured within OVS itself (case 3, above).
>>
>> Note: vlan can also be pushed or removed by the OpenFlow pipeline within
>> openvswitch between the ovs-port and the bond0.  So, there is actually no
>> reliable way to detect the correct set of vlan tags that should be used.
>> And also, even if the IP is assigned to the ovs-port that is part of the
>> same OVS bridge, there is no guarantee that packets routed to that IP can
>> actually egress from the bond0, as the forwarding rules inside the OVS
>>datapath can be arbitrarily complex.
>>
>> And all that is not limited to OVS even, as the cover letter mentions, TC
>> or nftables in the linux bridge or even eBPF or XDP programs are not that
>> different complexity-wise and can do most of the same things breaking the
>> assumptions bonding code makes.
>>
>>>
>>>       I understand that the architecture of OVS limits the ability to
>>> do these sorts of checks, but I'm unconvinced that implementing this
>>> support halfway is going to create more issues than it solves.

	Re-reading my comment, I clearly meant "isn't going to create
more issues" here.

>>>       Lastly, thinking out loud here, I'm generally loathe to add more
>>> options to bonding, but I'm debating whether this would be worth an
>>> "ovs-is-a-black-box" option somewhere, so that users would have to
>>> opt-in to the OVS alternate realm.
>
>> I agree that adding options is almost never a great solution.  But I had a
>> similar thought.  I don't think this option should be limited to OVS though,
>>as OVS is only one of the cases where the current verification logic is not
>>sufficient.

	Agreed; I wasn't really thinking about the not-OVS cases when I
wrote that, but whatever is changed, if anything, should be generic.

>What if we build on the arp_ip_target setting.  Allow for a list of vlan tags
> to be appended to each target. Something like: arp_ip_target=x.x.x.x[vlan,vlan,...].
> If a list of tags is omitted it works as before, if a list is supplied assume we know what were doing
> and use that instead of calling bond_verify_device_path(). An empty list would be valid.

	Hmm, that's ... not too bad; I was thinking more along the lines
of a "skip the checks" option, but this may be a much cleaner way to do
it.

	That said, are you thinking that bonding would add the VLAN
tags, or that some third party would add them?  I.e., are you trying to
accomodate the case wherein OVS, tc, or whatever, is adding a tag?

	-J

>>>
>>>       -J
>>>
>>>> Signed-off-by: David J Wilder <wilder@...ibm.com>
>>>> Signed-off-by: Pradeep Satyanarayana <pradeeps@...ux.vnet.ibm.com>
>>>> ---
>>>> drivers/net/bonding/bond_main.c | 8 +++++++-
>>>> 1 file changed, 7 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
>>>> index 950d8e4d86f8..6f71a567ba37 100644
>>>> --- a/drivers/net/bonding/bond_main.c
>>>> +++ b/drivers/net/bonding/bond_main.c
>>>> @@ -3105,7 +3105,13 @@ struct bond_vlan_tag *bond_verify_device_path(struct net_device *start_dev,
>>>>      struct net_device *upper;
>>>>      struct list_head  *iter;
>>>>
>>>> -    if (start_dev == end_dev) {
>>>> +    /* If start_dev is an OVS port then we have encountered an openVswitch
>>
>> nit: Strange choice to capitalize 'V'.  It should be all lowercase or a full
>> 'Open vSwitch' instead.
>
>>>> +     * bridge and can't go any further. The programming of the switch table
>>>> +     * will determine what packets will be sent to the bond. We can make no
>>>> +     * further assumptions about the network above the bond.
>>>> +     */
>>>> +
>>>> +    if (start_dev == end_dev || netif_is_ovs_port(start_dev)) {
>>>>              tags = kcalloc(level + 1, sizeof(*tags), GFP_ATOMIC);
>>>>              if (!tags)
>>>>                      return ERR_PTR(-ENOMEM);
>>>
>>> ---
>>>       -Jay Vosburgh, jv@...sburgh.net
>>
>> Best regards, Ilya Maximets.
>
>David Wilder (wilder@...ibm.com)

---
	-Jay Vosburgh, jv@...sburgh.net

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