netdev - Re: [ovs-dev] [PATCH net-next 2/2] net: openvswitch: add drop action

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Date: Wed, 12 Jul 2023 09:53:41 +0200
From: Adrian Moreno <amorenoz@...hat.com>
To: Aaron Conole <aconole@...hat.com>, Eric Garver <eric@...ver.life>
Cc: Ilya Maximets <i.maximets@....org>, Jakub Kicinski <kuba@...nel.org>,
 netdev@...r.kernel.org, dev@...nvswitch.org, Paolo Abeni
 <pabeni@...hat.com>, Eric Dumazet <edumazet@...gle.com>,
 "David S. Miller" <davem@...emloft.net>, Eelco Chaudron <echaudro@...hat.com>
Subject: Re: [ovs-dev] [PATCH net-next 2/2] net: openvswitch: add drop action



On 7/11/23 22:46, Aaron Conole wrote:
> Eric Garver <eric@...ver.life> writes:
> 
>> On Mon, Jul 10, 2023 at 06:51:19PM +0200, Ilya Maximets wrote:
>>> On 7/8/23 00:06, Jakub Kicinski wrote:
>>>> On Fri, 7 Jul 2023 18:04:36 +0200 Ilya Maximets wrote:
>>>>>>> That already exists, right? Johannes added it in the last release for WiFi.
>>>>>>
>>>>>> I'm not sure.  The SKB_DROP_REASON_SUBSYS_MAC80211_UNUSABLE behaves similarly
>>>>>> to that on a surface.  However, looking closer, any value that can be passed
>>>>>> into ieee80211_rx_handlers_result() and ends up in the kfree_skb_reason() is
>>>>>> kind of defined in net/mac80211/drop.h, unless I'm missing something (very
>>>>>> possible, because I don't really know wifi code).
>>>>>>
>>>>>> The difference, I guess, is that for openvswitch values will be provided by
>>>>>> the userpsace application via netlink interface.  It'll be just a number not
>>>>>> defined anywhere in the kernel.  Only the subsystem itself will be defined
>>>>>> in order to occupy the range.  Garbage in, same garbage out, from the kernel's
>>>>>> perspective.
>>>>>
>>>>> To be clear, I think, not defining them in this particular case is better.
>>>>> Definition of every reason that userspace can come up with will add extra
>>>>> uAPI maintenance cost/issues with no practical benefits.  Values are not
>>>>> going to be used for anything outside reporting a drop reason and subsystem
>>>>> offset is not part of uAPI anyway.
>>>>
>>>> Ah, I see. No, please don't stuff user space defined values into
>>>> the drop reason. The reasons are for debugging the kernel stack
>>>> itself. IOW it'd be abuse not reuse.
>>>
>>> Makes sense.  I wasn't sure that's a good solution from a kernel perspective
>>> either.  It's better than defining all these reasons, IMO, but it's not good
>>> enough to be considered acceptable, I agree.
>>>
>>> How about we define just 2 reasons, e.g. OVS_DROP_REASON_EXPLICIT_ACTION and
>>> OVS_DROP_REASON_EXPLICIT_ACTION_WITH_ERROR (exact names can be different) ?
>>> One for an explicit drop action with a zero argument and one for an explicit
>>> drop with non-zero argument.
>>>
>>> The exact reason for the error can be retrieved by other means, i.e by looking
>>> at the datapath flow dump or OVS logs/traces.
>>>
>>> This way we can give a user who is catching packet drop traces a signal that
>>> there was something wrong with an OVS flow and they can look up exact details
>>> from the userspace / flow dump.
>>>
>>> The point being, most of the flows will have a zero as a drop action argument,
>>> i.e. a regular explicit packet drop.  It will be hard to figure out which flow
>>> exactly we're hitting without looking at the full flow dump.  And if the value
>>> is non-zero, then it should be immediately obvious which flow is to blame from
>>> the dump, as we should not have a lot of such flows.
>>>
>>> This would still allow us to avoid a maintenance burden of defining every case,
>>> which are fairly meaningless for the kernel itself, while having 99% of the
>>> information we may need.
>>>
>>> Jakub, do you think this will be acceptable?
>>>
>>> Eric, Adrian, Aaron, do you see any problems with such implementation?
>>
>> I see no problems. I'm content with this approach.
> 
> +1

Sounds like a good plan. Thanks.

> 
>>> P.S. There is a plan to add more drop reasons for other places in openvswitch
>>>       module to catch more regular types of drops like memory issues or upcall
>>>       failures.  So, the drop reason subsystem can be extended later.
>>>       The explicit drop action is a bit of an odd case here.
>>>
>>> Best regards, Ilya Maximets.
>>>
>