Applies to SUSE Linux Enterprise Server 15 SP3

23 Managing kernel modules #

Although Linux is a monolithic kernel, it can be extended using kernel modules. These are special objects that can be inserted into the kernel and removed on demand. In practical terms, kernel modules make it possible to add and remove drivers and interfaces that are not included in the kernel itself. Linux provides several commands for managing kernel modules.

23.1 Listing loaded modules with lsmod and modinfo #

Use the lsmod command to view what kernel modules are currently loaded. The output of the command may look as follows:

> lsmod
Module                  Size  Used by
snd_usb_audio         188416  2
snd_usbmidi_lib        36864  1 snd_usb_audio
hid_plantronics        16384  0
snd_rawmidi            36864  1 snd_usbmidi_lib
snd_seq_device         16384  1 snd_rawmidi
fuse                  106496  3
nfsv3                  45056  1
nfs_acl                16384  1 nfsv3

The output is divided into three columns. The Module column lists the names of the loaded modules, while the Size column displays the size of each module. The Used by column shows the number of referring modules and their names. Note that this list may be incomplete.

To view detailed information about a specific kernel module, use the modinfo MODULE_NAME command, where MODULE_NAME is the name of the desired kernel module. Note that the modinfo binary resides in the /sbin directory that is not in the user's PATH environment variable. This means that you must specify the full path to the binary when running modinfo command as a regular user:

> /sbin/modinfo kvm
filename:       /lib/modules/5.3.18-57-default/kernel/arch/x86/kvm/kvm.ko.xz
license:        GPL
author:         Qumranet
suserelease:    SLE15-SP3
srcversion:     3D8FBA9060D4537359A06FC
depends:        irqbypass
supported:      yes
retpoline:      Y
intree:         Y
name:           kvm
vermagic:       5.3.18-57-default SMP mod_unload modversions

23.2 Adding and removing kernel modules #

While it is possible to use insmod and rmmod to add and remove kernel modules, it is recommended to use the modprobe tool instead. modprobe offers several important advantages, including automatic dependency resolution and blacklisting.

When used without any parameters, the modprobe command installs a specified kernel module. modprobe must be run with root privileges:

> sudo modprobe acpi

To remove a kernel module, use the -r parameter:

> sudo modprobe -r acpi

23.2.1 Loading kernel modules automatically on boot #

Instead of loading kernel modules manually, you can load them automatically during the boot process using the system-modules-load.service service. To enable a kernel module, add a .conf file to the /etc/modules-load.d/ directory. It is good practice to give the configuration file the same name as the module, for example:

/etc/modules-load.d/rt2800usb.conf

The configuration file must contain the name of the desired kernel module (for example, rt2800usb).

The described technique allows you to load kernel modules without any parameters. If you need to load a kernel module with specific options, add a configuration file to the /etc/modprobe.d/ directory instead. The file must have the .conf extension. The name of the file should adhere to the following naming convention: priority-modulename.conf, for example: 50-thinkfan.conf. The configuration file must contain the name of the kernel module and the desired parameters. You can use the example command below to create a configuration file containing the name of the kernel module and its parameters:

> echo "options thinkpad_acpi fan_control=1" | sudo tee /etc/modprobe.d/thinkfan.conf

Note: Loading kernel modules

Most kernel modules are loaded by the system automatically when a device is detected or user space requests specific functionality. Thus, adding modules manually to /etc/modules-load.d/ is rarely required.

23.2.2 Blacklisting kernel modules with modprobe #

Blacklisting a kernel module prevents it from loading during the boot process. This can be useful when you want to disable a module that you suspect is causing problems on your system. Note that you can still load blacklisted kernel modules manually using the insmod or modprobe tools.

To blacklist a module, add the blacklist MODULE_NAME line to the /etc/modprobe.d/50-blacklist.conf file. For example:

blacklist nouveau

Run the mkinitrd command as root to generate a new initrd image, then reboot your machine. These steps can be performed using the following command:

> su
echo "blacklist nouveau" >> /etc/modprobe.d/50-blacklist.conf && mkinitrd && reboot

To disable a kernel module temporarily only, blacklist it on-the-fly during the boot. To do this, press the E key when you see the boot screen. This drops you into a minimal editor that allows you to modify boot parameters. Locate the line that looks as follows:

linux /boot/vmlinuz...splash= silent quiet showopts

Add the modprobe.blacklist=MODULE_NAME command to the end of the line. For example:

linux /boot/vmlinuz...splash= silent quiet showopts modprobe.blacklist=nouveau

Press F10 or Ctrl–X to boot with the specified configuration.

To blacklist a kernel module permanently via GRUB, open the /etc/default/grub file for editing, and add the modprobe.blacklist=MODULE_NAME option to the GRUB_CMDLINE_LINUX command. Then run the sudo grub2-mkconfig -o /boot/grub2/grub.cfg command to enable the changes.