Chapter 22. Working with Kernel Modules

The Linux kernel is modular, which means it can extend its capabilities through the use of dynamically-loaded kernel modules. A Kernel module can provide:

a device driver which adds support for new hardware; or,
support for a file system such as btrfs orNFS.

Like the kernel itself, modules can take parameters that customize their behavior, though the default parameters work well in most cases. User-space tools can list the modules currently loaded into a running kernel; query all available modules for available parameters and module-specific information; and load or unload (remove) modules dynamically into or from a running kernel. Many of these utilities, which are provided by the module-init-tools package, take module dependencies into account when performing operations so that manual dependency-tracking is rarely necessary.

On modern systems, kernel modules are automatically loaded by various mechanisms when the conditions call for it. However, there are occasions when it is necessary to load and/or unload modules manually, such as when a module provides optional functionality, one module should be preferred over another although either could provide basic functionality, or when a module is misbehaving, among other situations.

This chapter explains how to:

use the user-space module-init-tools package to display, query, load and unload kernel modules and their dependencies;
set module parameters both dynamically on the command line and permanently so that you can customize the behavior of your kernel modules; and,
load modules at boot time.

Note: Installing the module-init-tools package

In order to use the kernel module utilities described in this chapter, first ensure the module-init-tools package is installed on your system by running, as root:

~]# yum install module-init-tools

For more information on installing packages with Yum, refer to Section 1.2.2, “Installing”.

22.1. Listing Currently-Loaded Modules

You can list all kernel modules that are currently loaded into the kernel by running the lsmod command:

~]$ lsmod
Module                  Size  Used by
xfs                   803635  1
exportfs                3424  1 xfs
vfat                    8216  1
fat                    43410  1 vfat
tun                    13014  2
fuse                   54749  2
ip6table_filter         2743  0
ip6_tables             16558  1 ip6table_filter
ebtable_nat             1895  0
ebtables               15186  1 ebtable_nat
ipt_MASQUERADE          2208  6
iptable_nat             5420  1
nf_nat                 19059  2 ipt_MASQUERADE,iptable_nat
rfcomm                 65122  4
ipv6                  267017  33
sco                    16204  2
bridge                 45753  0
stp                     1887  1 bridge
llc                     4557  2 bridge,stp
bnep                   15121  2
l2cap                  45185  16 rfcomm,bnep
cpufreq_ondemand        8420  2
acpi_cpufreq            7493  1
freq_table              3851  2 cpufreq_ondemand,acpi_cpufreq
usb_storage            44536  1
sha256_generic         10023  2
aes_x86_64              7654  5
aes_generic            27012  1 aes_x86_64
cbc                     2793  1
dm_crypt               10930  1
kvm_intel              40311  0
kvm                   253162  1 kvm_intel
[output truncated]

Each row of lsmod output specifies:

the name of a kernel module currently loaded in memory;
the amount of memory it uses; and,
the sum total of processes that are using the module and other modules which depend on it, followed by a list of the names of those modules, if there are any. Using this list, you can first unload all the modules depending the module you want to unload. For more information, refer to Section 22.4, “Unloading a Module”.

Finally, note that lsmod output is less verbose and considerably easier to read than the content of the /proc/modules pseudo-file.