Re: [PATCH 00/10 net-next] Convert CONFIG_IPV6 to built-in and remove stubs

[Fernando Fernandez Mancera <fmancera@suse.de>]

On 3/9/26 2:02 PM, Krzysztof Kozlowski wrote:
> On 09/03/2026 13:58, Daniel Borkmann wrote:
>> On 3/9/26 1:43 PM, Krzysztof Kozlowski wrote:
>>> On 09/03/2026 12:38, Fernando Fernandez Mancera wrote:
>>>> On 3/9/26 11:22 AM, Krzysztof Kozlowski wrote:
>>>>> On 09/03/2026 03:19, Fernando Fernandez Mancera wrote:
>>>>>> Historically, the Linux kernel has supported compiling the IPv6 stack as
>>>>>> a loadable module. While this made sense in the early days of IPv6
>>>>>> adoption, modern deployments and distributions overwhelmingly either
>>>>>> build IPv6 directly into the kernel (CONFIG_IPV6=y) or disable it
>>>>>> entirely (CONFIG_IPV6=n). The modular IPv6 use-case provides little to
>>>>>> no practical benefit today.
>>>>>
>>>>> It does. We all use generic kernels, thus it is one configuration for
>>>>> all boards and some setups have IPv6 and some not. The ones without IPv6
>>>>> just don't use that module.
>>>>
>>>> While I understand this, I would like to clarify that IMHO IPv6 isn't a
>>>> secondary protocol and it is fundamental to modern networking. This is
>>>
>>> Not for end user devices. None of my devices - neither routers, nor
>>> embedded boards, nor mobile phone from 5G provider - receive IPv6
>>> address, thus for them it is not fundamental.
>>
>> If that is the case for them, then they should just CONFIG_IPV6=n.
> 
> That's not a question to me. Look what I wrote: "We all use generic
> kernels" - in a meaning of generic kernel, with generic defconfig built
> once serving every machine.
> 
>>
>>> I agree it is fundamental for your cloud machines and network backbone
>>> which you are targeting, but this patchset completely ignores other
>>> users calling their use-cases "little practical benefit"! Try running
>>> Amiga machine...
>>
>> Are you talking about [0]? That's legacy hardware and in this case just
> 
> Dunno what is legacy there...
> 
>> disable IPv6 altogether, why would you still prefer to have it as a module?
> 
> That's not a question to me - someone added it as a module now. The
> author here changes it to built-in.
> 
> 
>>
>>     [0] https://en.wikipedia.org/wiki/Amiga
>>
>>> There is no even bloatometer stats for these defconfigs so we can see
>>> the impact.
>>>
>>>> why I believe it should be built-in by default. Currently OpenWRT,
>>>> Debian ARM and others already ships the kernel with CONFIG_IPV6=y. I
>>>> know that Alpine and Yocto doesn't do that for arm64.
>>>
>>> Maybe there are other reasons why distro should not choose it as module
>>> (like module load calls on ipv6 packets) but that was not explained here.
>>>
>>> Arch Linux Rpi kernel on arm64 has IPV6=6 and the module itself is 650
>>> kB. That's noticeable for smaller arm64 boards.
>>>
>>> For arm it would be even more noticeable as some have 256 MB RAM like
>>> first Rpi.
>>
>> You are arguing that these will never be able to migrate to an IPv6 world
>> given their memory is too small?
> 
> No. I argue that they do not need IPv6 in many cases, thus use-case of
> IPV6=m is perfectly valid.
> 
> The entire discussion here started with "The modular IPv6 use-case
> provides little to no practical benefit today."
> 
> and this is clearly false. I brought you already few arguments of valid
> use case today.
> 
>>
>>>> I guess the most critical one here is Yocto but if the developer of the
>>>> embedded device is sure they won't use IPv6 at all, they should turn it off.
>>>>
>>>> At the same time, Alpine ships software that enable IPV6 and is
>>>> frequently loaded as a module. So the only remaining concern would be
>>>> the boot partition size. I don't really have a solution for that problem.
>>>>
>>>> I think that the infrastructure for allowing IPV6=m is bug-prone and it
>>>> impacts performance. Forcing the use of indirect function calls in core
>>>> networking, Netfilter or BPF datapaths seems like a heavy tax to me.
>>>>
>>>>> Also, with these generic kernels (so again all machines are using same
>>>>> ones, e.g. distro) users can easily blacklist the module.
>>>>
>>>> FWIW; users can still boot with kernel command line parameter
>>>> ipv6.disable=1 and then IPV6 will be administratively disabled.
>>>
>>> It's not the same. You enabled it on amiga_defconfig and do you
>>> understand what sort of machine is that? The newest have 16 MB RAM, many
>>> much less like 2 MB, and ipv6 module on m68k is 400 kB, so pretty
>>> significant change.
>>
>> If IPv6 is not relevant for amiga_defconfig presumably it should just be
>> set to CONFIG_IPV6=n?
> 
> It might be relevant to some, but this makes it relevant to everyone on
> Amiga.
> 
> There is a reason why kernel supports modules or do you suggest "modules
> provide little to no practical benefit today" and let's just have
> everything built-in or disabled.
> 

Right, maybe that is a too bold statement. Probably it could be 
rephrased to:

"While maintaining a modular IPv6 stack still offers footprint savings 
for specific setups, this benefit is outweighed by the architectural 
burden it imposes on the rest of the kernel."

I believe that having modules is useful. I just do not think it pays off 
here.