Re: [gentoo-user] Kernel modules: initramfs vs. /lib/modules

[Rich Freeman]

On Fri, Feb 10, 2017 at 6:58 AM, marco restelli <mrestelli@×××××.com> wrote:
> Hi all,
>    I am trying to understand a bit initramfs and genkernel and I have
> few (basic) questions.
>
> I understand that one must have in the initramfs those modules which
> are required to boot the system, for instance to access /dev . Now:

Sort-of.  You need an initramfs if the kernel cannot otherwise mount
/, and /usr (if it isn't on the same filesystem as /).  Being able to
mount / is an absolute requirement, there are other ways to go about
mounting /usr.

An initramfs has some benefits even if the kernel could mount /, such
as making it easier or more reliable to identify the correct root
filesystem.

>
> - can a module be present both in the initramfs and as kernel module
>   in /lib/modules ?
>

Yes, and typically all of the initramfs modules are present in both.

> - how does genkernel decide which modules to put in the initramfs ?

I can't speak to genkernel specifically, but most initramfs generators
include all modules.  Other than space and miniscule load time there
isn't much reason not to.

>
> - can modules included in the initramfs be unloaded once the system is
>   running, as  modprobe -r

Yes, assuming it isn't in use.  Most of the stuff loaded by the
initramfs is probably going to be in use until you shut down (such as
the module for the root filesystem).

>
> - can modprobe load modules from the initramfs ?
>

Only if it is run from within the initramfs.  Otherwise this is like
asking whether a binary in a chroot can run something outside the
chroot.  Of course, typically all the initramfs modules are also
present in /lib so modprobe will just load the module from there.

>
> Well, clearly I am a bit confused about the topic - I hope somebody
> can help me a bit!
>

An initramfs is really just a chroot in some sense.  Though, it would
be more accurate to say that the system you're using after you've
booted is the chroot, and the initramfs was the original host.  The
initramfs is the root filesystem when the kernel boots, and it
basically does whatever it needs to to find the real root filesystem,
mount it, and then it deletes its stuff to free up ram, chroots to the
real root, and execs the real init.  At that point very little of the
initramfs is left, other than any kernel modules it might have loaded
(which are no different from kernel modules loaded at a later point in
time).

It is just a way to do userspace bootstrapping.  Coreboot/libreboot
take this to yet another level.  Rather than try to build the smarts
into the kernel to handle every conceivable system configuration, the
kernel provides the driver and some basic logic, and if you want to do
something fancier you use an initramfs and the initramfs can do
anything you can do in linux userspace to find and mount root.  It
could download root from a webserver, or launch postfix and wait for
somebody to send the root filesystem as an attachment, or whatever
your imagination can come up with.

Usually, though, it ends up just mounting /dev/sda2 or whatever.  Most
distros use an initramfs by default because it is more robust and can
handle things like UUIDs and labels.  That way if you plug in a new
drive and your existing drives get renumbered the correct filesystem
gets mounted.  That, and it lets them use highly modular kernels
without having to know what kind of filesystem you'll use for /, since
it can just be modprobed at run time.  This lets them build all the
drivers as modules, which costs some disk space and a lot of one-time
compile time, but gives the end-user more flexibility without any need
to custom-build a kernel.  Gentoo is a bit unusual in encouraging
users to build their own kernels, but of course once you do that then
there is no need to build all the drivers, or use an initramfs for
modules needed to mount /.

Otherwise, there is nothing special about modules loaded from the
initramfs.  They're just kernel modules.

-- 
Rich