Chapter 1. Working with kernel modules

This Chapter explains:

  • What is a kernel module.
  • How to use the kmod utilities to manage modules and their dependencies.
  • How to configure module parameters to control behavior of the kernel modules.
  • How to load modules at boot time.
Note

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

# yum install kmod

1.1. What is a kernel module?

The Linux kernel is monolithic by design. However, it is compiled with optional or additional modules as required by each use case. This means that you can extend the kernel’s capabilities through the use of dynamically-loaded kernel modules. A kernel module can provide:

  • a device driver which adds support for new hardware,
  • support for a file system such as GFS2 or NFS.

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 kmod 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 needed. However, there are occasions when it is necessary to load or unload modules manually. For example when one module is preferred over another although either is able to provide basic functionality, or when a module is misbehaving.

1.2. Listing currently-loaded modules

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

# lsmod
Module                  Size  Used by
tcp_lp                 12663  0
bnep                   19704  2
bluetooth             372662  7 bnep
rfkill                 26536  3 bluetooth
fuse                   87661  3
ebtable_broute         12731  0
bridge                110196  1 ebtable_broute
stp                    12976  1 bridge
llc                    14552  2 stp,bridge
ebtable_filter         12827  0
ebtables               30913  3 ebtable_broute,ebtable_nat,ebtable_filter
ip6table_nat           13015  1
nf_nat_ipv6            13279  1 ip6table_nat
iptable_nat            13011  1
nf_conntrack_ipv4      14862  4
nf_defrag_ipv4         12729  1 nf_conntrack_ipv4
nf_nat_ipv4            13263  1 iptable_nat
nf_nat                 21798  4 nf_nat_ipv4,nf_nat_ipv6,ip6table_nat,iptable_nat
[output truncated]

The lsmod output specifies three columns:

  • Module

    • The name of a kernel module currently loaded in memory.

  • Size

    • The amount of memory the kernel module uses in kilobytes.

  • Used by

    • A decimal number representing how many dependencies there are on the Module field.
    • A comma separated string of dependent Module names. Using this list, you can first unload all the modules depending the module you want to unload.

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

1.3. Displaying information about a module

You can display detailed information about a kernel module using the modinfo module_name command.

Note

When entering the name of a kernel module as an argument to one of the kmod utilities, do not append a .ko extension to the end of the name. Kernel module names do not have extensions; their corresponding files do.

Example 1.1. Listing information about a kernel module with lsmod

To display information about the e1000e module, which is the Intel PRO/1000 network driver, enter the following command as root: 

# modinfo e1000e
filename:       /lib/modules/3.10.0-121.el7.x86_64/kernel/drivers/net/ethernet/intel/e1000e/e1000e.ko
version:        2.3.2-k
license:        GPL
description:    Intel(R) PRO/1000 Network Driver
author:         Intel Corporation,