netdev - Re: [REGRESSION]: mlx5_core does not autoload in Linux 6.18

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Message-ID: <2af9d46e-647c-44a7-8c19-32a749408130@gmail.com>
Date: Thu, 4 Dec 2025 22:21:29 -0500
From: Demi Marie Obenour <demiobenour@...il.com>
To: Thorsten Leemhuis <regressions@...mhuis.info>,
 Saeed Mahameed <saeedm@...dia.com>, Tariq Toukan <tariqt@...dia.com>,
 Mark Bloch <mbloch@...dia.com>, netdev <netdev@...r.kernel.org>
Cc: regressions@...ts.linux.dev
Subject: Re: [REGRESSION]: mlx5_core does not autoload in Linux 6.18

On 12/4/25 06:53, Thorsten Leemhuis wrote:
> Lo! Thx for the report.
> 
> Let me CC the maintainers of said driver, maybe they have an idea or
> even head about the problem already. If not, you might need to bisect this.

Interestingly, the problem does not reproduce after a reboot and
a further Nixpkgs upgrade.  This either means that NixOS failed to
regenerate the initramfs, or that the problem is not reproducible
reliably.

> On 12/4/25 08:54, Demi Marie Obenour wrote:
>> In Linux 6.18, mlx5_core does not load automatically.  This causes
>> my server to not be accessible from the network.  Manually loading
>> the module in the initramfs fixes the problem.  Everything worked
>> with Linux 6.17.x.
>>
>> I'm using the linuxPackages_latest package from Nixpkgs.
> 
> Mentioning it like that might scare kernel developers, as they have no
> idea what this is and thus might suspect that it's a vendor kernel with
> lots of patches applied. Is that the case? Or is that vanilla or at
> least close to vanilla?

https://github.com/NixOS/nixpkgs/tree/7905606cfa51a1815787377b9cb04291e87ebcb4/pkgs/os-specific/linux/kernel
shows that there are very few patches.  The only ones I see are:

- Change the RANDSTRUCT seed to one provided by NixOS.
- Replace hard-coded paths with ones that are present in NixOS.
- Make Kconfig print ### after every question.
- Change two EXPORT_SYMBOL_GPL to EXPORT_SYMBOL
  so that ZFS works with PREEMPT_RT.

Of those, the first one is needed for reproducible builds with
RANDSTRUCT.  The second is needed because of NixOS's filesystem
structure, which isn't FHS-conforming.  The third is used by
NixOS-specific automation, and the last one is clearly not upstreamable
but also will not break anything.

>>  The server is a RISE-7 OVH dedicated server with an AMD Epyc 7402 CPU.
> 
> Ciao, Thorsten
> 
> #regzbot ^introduced: v6.17..v6.18
> #regzbot title: net: mlx5_core: module does not autoload anymore Linux 6.18
I wonder if this is even a kernel bug.  It could be that NixOS
generated a broken userspace that couldn't load the module.
The problem going away after nixos-rebuild supports this.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
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