netdev - Re: [PATCH net-next 2/2] bonding: make hard-coded defines configurable at build

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Date:	Tue, 15 Jul 2014 10:33:25 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Veaceslav Falico <vfalico@...il.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Jay Vosburgh <j.vosburgh@...il.com>,
	Andy Gospodarek <andy@...yhouse.net>
Subject: Re: [PATCH net-next 2/2] bonding: make hard-coded defines
 configurable at build

On Tue, Jul 15, 2014 at 10:11 AM, Veaceslav Falico <vfalico@...il.com> wrote:
> On Tue, Jul 15, 2014 at 09:45:36AM -0700, Alexei Starovoitov wrote:
>>
>> On Tue, Jul 15, 2014 at 9:18 AM, Veaceslav Falico <vfalico@...il.com>
>> wrote:
>>>
>>> On Tue, Jul 15, 2014 at 08:48:01AM -0700, Alexei Starovoitov wrote:
>>>>
>>>>
>>>> On Tue, Jul 15, 2014 at 3:15 AM, Veaceslav Falico <vfalico@...il.com>
>>>> wrote:
>>>>>
>>>>>
>>>>> +
>>>>> +config BOND_MAX_VLAN_ENCAP
>>>>> +       int "Maximum number of stacked vlans on top of bonding"
>>>>> +       default "2"
>>>>> +
>>>>
>>>>
>>>> I don't think we should allow changing these defaults so easily.
>>>> Not a single HW supports 3 vlan tags. There is no standard for it
>>>> either.
>>>> Why you would ever change this?
>>>
>>>
>>> There have been discussions about vlan nestings for bonding, and the
>>> outcome was that more than 2 are possible. Also, iirc, no standard limits
>>> it to only 2.
>>
>>
>> standard doesn't say that the maximum is 2, but it doesn't specify what
>> should be done in such case, so all vlan-aware switches that I know will
>> be just dropping packets with 3 vlans.
>
>
> There might be switches that support it/don't really care, there are
> bridges (including soft ones) etc. that all can make use of it.
>
> Given that it's a build-time configuration option that affects a small part
> of arp monitoring, which is a small part of bonding module, which is ... -
> I don't understand why it can't be configured.

For 3 vlan case to be useful, first somebody needs to define a meaning
for it and real use case. I haven't seen one.
Making bond driver support fictitious configuration is pointless.

>> Therefore for bond driver there is no reason to accept such packets
>> in the first place from user space, since they won't go too far in the
>> network.
>>
>>> These defaults are scalable by their nature, and there are people
>>> maintaining their own patches to change them. So making them available to
>>> be configured at compile time is a good thing to do, I think.
>>
>>
>> people keep a patch to support 3 vlans? what's the use case?
>
>
> I guess it has something to do with virtualization, including nested one.

sounds like you're inventing a use case instead of having it already.
imo we shouldn't be adding features because it _feels_ useful.
use cases gotta be real.

> But, again, does this matter? I don't see how it can give us something bad.
> It's a configuration option with a default value that suits most users, and
> that might be configured for some advanced/weird use-cases.

it's bad, since it increases test matrix.
You said there are people out there that have some secret patches
to tweak these defaults. Please share.
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