kernel-module-configuration-service for configuring kernel parameters

Hi raid5atemyhomework,

Hi everyone,

@raid5atemyhomework, thanks for all the patches!

On Wed, 06 Jan 2021 15:57:19 +0000

raid5atemyhomework via Guix-patches via <guix-patches@gnu.org> wrote:

Sure, but it would be better to create a way to configure those module parameters

in Guix in a declarative way, first.

Your new kernel-loadable-module-service-type would be a good template

to write a kernel-module-configuration-service that can be extended by

other services. The latter should allow users to parametrize the kernel

in general via the operating-system form, and also allow services to extend

such configuration (with merging, and conflict detection).

It would be used:

(1) To declaratively specify the contents of something like /etc/modprobe.d .

It shouldn't even be called "/etc/modprobe.d"--it should also be in the store

instead. This directory is only useful for non-Linux-builtins.

(2) If the "module" is built-in then the kernel command line must get the

options instead. (in fact, it works as a kernel command line option also

if it's a loadable module--so not sure we need /etc/modprobe.d at all--at

least at first. But there's probably a maximal length for the kernel command

line that we could exceed if we did that long term)

I know it's annoying that Guix doesn't have this facility already, but the

time to introduce an interface for /etc/modprobe.d and the kernel command

line for builtin modules is before other services introduce their own

ad hoc way to create /etc/modprobe.d--like this tries to do here.

See also https://issues.guix.info/issue/42193for an earlier attempt (which

is already very far--but it has a bug somewhere). There's also already a

kernel profile thing like you wrote in that patchset.

(Note that I would prefer there not to be a "LOAD?" in there because it

confuses loading the module (which is usually NOT started by user space

but by the kernel on its own) and confguring the module (which has to be

done by user space because it's specifying policy, not mechanism))

Also, because the kernel usually loads loadable modules on its own (potentially

really early), /etc/modprobe.d has to be preset and known to the modprobe

executable VERY EARLY (via environment variable MODPROBE_OPTIONS--see

gnu/services.scm %modprobe-wrapper).

It is totally possible that some modules in the initrd need options, too

(see load-linux-modules-from-directory for where this would need to go).

load-linux-module/fd already accepts options and flags--but both are not given

on the call. For this, part of future kernel-module-configuration entries

(the ones needed for modules in the initrd) should be copied into the initrd,

too.

Then there's the handoff between initrd and main system. It would be bad

if the kernel tried and succeeded to load a module that is not in the initrd

just before the modprobe.d directory is set up (because it would be loaded

without passing the options the user configured)--so that needs to be avoided.

@ludo: Could you help here?

Note:

This can be usefully put in a modprobe.d-like directory if zfs is a module, but

not if it's built into the kernel. But it can be put into the kernel command

line in both cases.

But I guess the ZFS Linux kernel module can't be built-in into the kernel

anyway.

But that's a special case--in general, it's very much possible to make modules

built-in.

Definitely.

Yes--but why is that dependent on 'file-systems ? Is it because it needs /proc ?

Or is it an oversight ? I would prefer to get rid of this dependency and then

use kernel-module-loader-service-type.

Also, this manual loading of kernel modules is not supposed to be the way to

do things in Linux. That a kernel module was compiled as a module is

an *implementation detail*--so Linux should (and usually does) automatically

load kernel modules the first time a device for them is accessed (after all,

how would user space know whether something is compiled as a module or

built-in--that would be too much to ask).

Linux is not a microkernel, so the kind of modularily modprobe.d suggests

exists does not in fact exist in kernel space--even though Linux does a good

job faking it: modprobe.d contains:

aliases allowed!)

"fun" to map to Guix)

module.

module.

not do what one would intuitively think!).

If the file name of the regular file under /etc/modprobe.d is not used for

anything, then we can just have one file /gnu/store/*modprobe.d/guix.conf

in total in there.

Then there are sysctl kernel parameters--but those Guix already exposes via

sysctl-service-type. But those should also be made able to be extended

by other services, and merge conflicts should be handled. For example, users

often set net.ipv4.ip_forward=1 (for example via sysctl).

Thank you for all your effort to make ZFS work nicely in Guix.

Hello Danny,

Looks like that patchset was merged in, so basically I can just depend on that? So the first patch in this patchset would be dropped?

ZFS *can* be built-in to the kernel, Ubuntu does it. You can't distribute it like that (Ubuntu distributes it like that but presumably they have enough lawyers to muddy the waters so that they can get away with it), but as the documentation in this patch notes: the user has every right to do whatever they want on the machine they own, including build a Linux kernel that has ZFS built-in and run it, they just can't make that version available to somebody else.

So to go whole-hog, we would have a service that replaces the kernel package and inserts kernel module sources in-tree somehow, then compiles Linux-libre on the user's machine. That would probably be a lot more painful to install ZFS with (the user has to recompile the whole kernel at each update of either kernel or ZFS, whereas with a kernel module the user has to recompile just the kernel module), so maybe kernel module is still better overall.

Dunno --- one VM I tested, I removed the `zfs-scan-automount` shepherd service from the `file-systems` target, and the VM still wouldn't boot, claiming a stack overflow (the same error which I got when I was still trying to use kernel-module-loader-service-type here). Or maybe I just got confused with which VM was which, testing VMs wasn't a stress-free vacation. I just want ZFS, because MD RAID5 ate my homework, this is getting tiresome...

One thing I notice about `kernel-module-loader-service-type` is that it's not instantiated in essential services, or indeed anywhere in Guix. A few services *do* extend it. But my **very rough** understanding is that if you're going to extend a service, it had better be instantiated *once* in the list of services.

In particular I note that the documentation for `kernel-module-loader-service-type` shows an example where it uses `service` to program the `kernel-module-loader-service-type`, not `simple-service`. This suggests to me that `kernel-module-loader-service-type` is broken because it's not in the list of essential services but is extensible. Maybe. It's designed as an extensible service, but isn't instantiated at default. Maybe that's what really bit me and not the shepherd circular dependency loop? *shrug*

So how do I get ZFS loaded? Note that the devices it targets are block devices and it needs to scan for block devices that are formatted for ZFS. Do other filesystems have some autoload rule?

Thanks

raid5atemyhomework

merge 45692 45703

quit

merge 45692 45643

quit

close 45692

quit

No point: nobody will review or merge anyway.