21 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.
21.1 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:
tux > 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/4.12.14-94.37-default/kernel/arch/x86/kvm/kvm.ko license: GPL author: Qumranet srcversion: BDFD8098BEEA517CB75959B depends: irqbypass intree: Y vermagic: 4.4.57-18.3-default SMP mod_unload modversions signer: openSUSE Secure Boot Signkey sig_key: 03:32:FA:9C:BF:0D:88:BF:21:92:4B:0D:E8:2A:09:A5:4D:5D:EF:C8 sig_hashalgo: sha256 parm: ignore_msrs:bool parm: min_timer_period_us:uint parm: kvmclock_periodic_sync:bool parm: tsc_tolerance_ppm:uint parm: lapic_timer_advance_ns:uint parm: halt_poll_ns:uint parm: halt_poll_ns_grow:int parm: halt_poll_ns_shrink:int
21.2 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:
tux > sudo modprobe acpi
To remove a kernel module, use the -r parameter:
sudo modprobe -r acpi
21.2.1 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
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.
21.2.2 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.