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

[Krzysztof Kozlowski <krzk@kernel.org>]

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.

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...

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.

> 
> 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.

Best regards,
Krzysztof