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

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Message-ID: <20140715171139.GD5225@mikrodark.usersys.redhat.com>
Date:	Tue, 15 Jul 2014 19:11:39 +0200
From:	Veaceslav Falico <vfalico@...il.com>
To:	Alexei Starovoitov <alexei.starovoitov@...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 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.

>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.

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.
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