[dm-devel] [PATCH 5/6] multipath: Makefile: modules-load.d file for SCSI device handlers

[Martin Wilck]

On Wed, 2022-02-02 at 18:37 -0600, Benjamin Marzinski wrote:
> On Wed, Feb 02, 2022 at 12:40:23AM +0100, mwilck at suse.com wrote:
> > From: Martin Wilck <mwilck at suse.com>
> > 
> > The kernel's autoload mechanism for SCSI device handlers doesn't
> > work during SCSI device probing. While it's possible to load and
> > attach device handlers after probing, it has disadvantages: the
> > handlers are useful for error handling even before multipathd has
> > started, and at least up to kernel 5.17, the sysfs "access_state"
> > attribute
> > will remain invisible for already probed devices.
> > 
> > Distributions will therefore want to make sure the handlers are
> > either
> > built-in in the kernel, or loaded early. Add functionality to
> > create and install a modules-load.d file with a list of handlers
> > to load. By default, the list is empty, and no file will be
> > generated.
> > The list can be specified at install-time like this:
> > 
> >    make SCSI_DH_MODULES_PRELOAD="scsi_dh_rdac scsi_dh_emc" install
> > 
> > dracut automatically adds modules-load.d files and the modules they
> > reference to the initramfs.
> > 
> > Note: distributions that compile scsi_mod as a module may rather
> > want
> > to use a modprobe.d file with a statement like this:
> > 
> > softdep scsi_mod post: scsi_dh_alua scsi_dh_rdac
> > 
> 
> I think you might need a default scsi_dh.conf file with a comment
> header
> or something. install won't install a file that doesn't exist. Or am
> I
> missing something here?

Yes, you would need a conf file. But we can't construct it in the
upstream code. It depends on how the distro compiles the SCSI modules.
softdeps don't work with built-in modules, and with current kmod, there
can be only one softdep directive per module (no stacking) [1]; thus if
the distro (or the admin) needs further softdeps for scsi_mod, the
directives would need to be merged into one, which is tricky to
automate.

The advantage of the softdep over modules-load.d is that it avoids
loading the device handlers for systems without SCSI (like NVMe
laptops, or embedded devices). The distro needs to decide whether or
not that's worth it.

On the contrary, the modules-load.d file stacks nicely with other
packages.

Regards,
Martin

[1] https://www.spinics.net/lists/linux-modules/msg01654.html