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Table of Contents
The Linux Kernel
What Does the Kernel Do?
You've probably heard people talking about compiling the kernel or building a kernel, but what exactly is the kernel and what does it do? The kernel is the center of your computer. It is the foundation for the entire operating system. The kernel acts as a bridge between the hardware and the applications. This means that the kernel is (usually) the sole piece of software responsible for ordering around the hardware components of your computer. It is the kernel that instructs the hard drive to search for a certain data stream. It is the kernel that instructs your network card to transmit rapid changes in voltage. The kernel also listens to hardware as well. When the network card detects a remote computer sending information, it forwards that information to the kernel. This makes the kernel both the single most important piece of software on your computer and the most complex.
Working with Modules
The complexity of a modern linux kernel is staggering. The source code for the kernel weighs in at nearly 400MB uncompressed. There are thousands of developers, hundreds of options, and if everything were built together, the kernel would soon pass 100MB in size itself. In order to keep the size of the kernel down (as well as the amount of RAM needed for the kernel), most of the kernel options are built as modules. You can think of these modules as device drivers which can be inserted or removed from a running kernel at will. In truth, many of them aren't device drivers at all, but contain support for things such as network protocols, security measures, and even filesystems. In short, nearly any piece of the linux kernel can be built as a loadable module.
It's important to realize that Slackware will automatically handle
loading most modules for you. When your system boots,
udevd(8) is started and begins to probe your
system's hardware. For each device it finds, it loads the proper module
and created a device node in
/dev. This usually
means that you will not need to load any modules in order to use your
computer, but occasionally this is necessary.
So what modules are currently loaded on your computer and how do we load and unload them? Fortunately we have a full suite of tools for handling this. As you might have guessed, the tool for listing modules is lsmod(8).
darkstar:~# lsmod Module Size Used by nls_utf8 1952 1 cifs 240600 2 i915 168584 2 drm 168128 3 i915 i2c_algo_bit 6468 1 i915 tun 12740 1 ... many more lines ommitted ...
In addition to showing you what modules are loaded, it displays the size of each module and tells you what other modules are using it.
There are two applications for loading modules: insmod(8) and modprobe(8). Both will load modules and report any errors (such as loading a module for a device that isn't present in your system), but modprobe is preferred because it can load any module dependencies. Using either is straight-forward.
darkstar:~# insmod ext3 darkstar:~# modprobe ext4 darkstar:~# lsmod | grep ext ext4 239928 1 jbd2 59088 1 ext4 crc16 1984 1 ext4 ext3 139408 0 jbd 48520 1 ext3 mbcache 8068 2 ext4,ext3
Removing modules can be a tricky process, and once again we have two programs for removing them: rmmod(8) and modprobe. In order to remove a module with modprobe, you'll need to use the -r argument.
darkstar:~# rmmod ext3 darkstar:~# modprobe -r ext4 darkstar:~# lsmod | grep ext
Compiling A Kernel and Why to do So
Most Slackware users will never need to compile a kernel. The huge and generic kernels contain virtually all the support you will need.
However, some users may need to compile a kernel. If your computer contains bleeding edge hardware, a newer kernel may offer improved support. Sometimes a kernel patch may be available that corrects a problem you are experiencing. In these cases a kernel compile is probably warranted. Users who simply want the latest and greatest version or who believe using a custom compiled kernel will give them greater performance can certainly upgrade, but are unlikely to actually notice any major changes.
If you still think compiling your own kernel is something you want or need to do, this section should walk you through the many steps. Compiling and installing a kernel is not that difficult, but there are a number of mistakes that can be made along the way, many of which can prevent your computer from booting and cause major frustration.
The first step is ensuring you have the kernel source code installed
on your system. The kernel source package is included in the
“k” disk set in the Slackware installer, or you can download
another version from http://www.kernel.org/.
Traditionally, the kernel source is located in
/usr/src/linux, a symbolic link that
points to the specific kernel release used, but this is by no means
set in stone. You can place the kernel source code virtually anywhere
without encountering any problems.
darkstar:~# ls -l /usr/src lrwxrwxrwx 1 root root 14 2009-07-22 19:59 linux -> linux-220.127.116.11/ drwxr-xr-x 23 root root 4096 2010-03-17 19:00 linux-18.104.22.168/
The most difficult part of any kernel compile is the kernel
configuration. There are hundreds of options, many of which can
optionally be compiled into modules. This means there are thousands of
ways to configure a kernel. Fortunately, there are a few handy tricks
that can keep you from running into too much trouble. The kernel
configuration file is
.config. If you are very
brave, you can manually edit this file with a text editor, but I highly
recommend you use the kernel's built-in tools for manipulating
Unless you are very familiar with configuring kernels, you should
always start with a solid base configuration and modify it. This
prevents you from skipping an important option that might force you to
compile the kernel again and again until you get it right. The best
.config files to start with are those used
by Slackware's default kernels. You can find them on your Slackware
install disks or at your favorite mirror in the
darkstar:~# mount /mnt/cdrom darkstar:~# cd /mnt/cdrom/kernels darkstar:/mnt/cdrom/kernels# ls VERSIONS.TXT huge.s/ generic.s/ speakup.s/ darkstar:/mnt/cdrom/kernels# ls genric.s System.map.gz bzImage config
You can replace the default
.config file easily by
copying or downloading the
config file for the
kernel you wish to use as a base. Here I am using Slackware's
recommended generic.s kernel for a base, but you may wish to use the
huge.s config file. The generic kernel builds more things as modules
and thus creates a smaller kernel image, but it usually requires the
use of an initrd.
darkstar:/mnt/cdrom/kernels# cp generic.s/config /usr/src/linux/.config
The Slackware kernel file lacks the “dot” while the kernel file
includes it. If you forget, or simply copy the
.config file was already present will be used
If you want to use the configuration for the currently running kernel
as your base, you may be able to locate it at
/proc/config.gz. This is a special kernel-related
file that includes the entire kernel configuration in a compressed
format and requires that your kernel was built to support it.
darkstar:~# zcat /proc/config.gz > /usr/src/linux/.config
Now that we've created a solid base configuration, it's time to make any configuration changes we want. The entire kernel build process from configuration to compilation is performed with the make(1) command and special arguments to it. Each argument performs a different function.
If you are upgrading to a newer kernel release, you will definitely
want to use the oldconfig argument. This will step through
.config and look for missing elements
that usually indicates that the new kernel release contains additional
options. Since options are added at virtually every kernel release,
this is generally a good thing to do.
darkstar:/usr/src/linux# make oldconfig scripts/kconfig/conf -o arch/x86/Kconfig * * Restart config... * * * File systems * Second extended fs support (EXT2_FS) [M/n/y/?] m Ext2 extended attributes (EXT2_FS_XATTR) [N/y/?] n Ext2 execute in place support (EXT2_FS_XIP) [N/y/?] n Ext3 journalling file system support (EXT3_FS) [M/n/y/?] m Ext3 extended attributes (EXT3_FS_XATTR) [Y/n/?] y Ext3 POSIX Access Control Lists (EXT3_FS_POSIX_ACL) [Y/n/?] y Ext3 Security Labels (EXT3_FS_SECURITY) [Y/n/?] y The Extended 4 (ext4) filesystem (EXT4_FS) [N/m/y/?] (NEW) m
Here you can see that I the new kernel I am compiling has added support for a new filesystem: ext4. oldconfig has gone through my original configuration, kept all the old options exactly as they were set, and prompted me on what to do with new options. Typically it is save to choose the default option, but you may wish change this. oldconfig is a very handy tool for presenting you with only new configuration options, making it ideal for users who simply have to try out the latest kernel release.
For more serious configuration tasks, there are a multitude of options. The linux kernel can be configured in three primary ways. The first is config, which will step through each and every option one by one and ask what you would like to do. This is so tedious that hardly anyone ever uses it anymore.
darkstar:/usr/src/linux# make config scripts/kconfig/conf arch/x86/Kconfig * * Linux Kernel Configuration * * * General setup * Prompt for development and/or incomplete code/drivers (EXPERIMENTAL) [Y/n/?] Y Local version - append to kernel release (LOCALVERSION)  -test Automatically append version information to the version string (LOCALVERSION_AUTO) [N/y/?] n Support for paging of anonymous memory (swap) (SWAP) [Y/n/?]
Fortunately, there are two much easier ways to configure your kernel, menuconfig and xconfig. Both of these create a menu-driven program that lets you select and de-select options without having to step through each one. menuconfig is the most commonly used method, and the one I recommend. xconfig is only useful if you are attempting to compile the kernel from a graphical user interface within X. Both are so similar however, that we are only going to document menuconfig.
Running make menuconfig from a terminal will present you with the friendly curses-driven interface you see below. Each kernel section is given its own submenu, and you can navigate with the arrow keys.
If you are compiling a kernel that is the same release as the stock Slackware kernel, you must set the “Local version” option. This is found on the “General setup” submenu. Failure to set this will result in your kernel compile over-writing all the modules used by the stock kernels. This can quickly render your system unbootable.
Once you've finished configuring the kernel, it's time to begin compiling it. There are many different methods for this, but the most reliable is to use bzImage. When you pass this argument to make, the kernel compilation will begin and you will see lots of data scroll through the terminal until either the compile process is complete or a fatal error is encountered.
darkstar:/usr/src/linux# make bzImage scripts/kconfig/conf -s arch/x86/Kconfig CHK include/linux/version.h CHK include/linux/utsrelease.h SYMLINK include/asm -> include/asm-x86 CALL scripts/checksyscalls.sh CC scripts/mod/empty.o HOSTCC scripts/mod/mk_elfconfig MKELF scripts/mod/elfconfig.h HOSTCC scripts/mod/file2alias.o ... many hundreds of lines ommitted ...
If the process ends in an error, you should check your kernel
configuration first. Compile errors are usually caused by a fault
.config file. Assuming everything went alright,
we're still not entirely finished, as we need to build the modules.
darkstar:/usr/src/linux# make modules CHK include/linux/version.h CHK include/linux/utsrelease.h SYMLINK include/asm -> include/asm-x86 CALL scripts/checksyscalls.sh HOSTCC scripts/mod/file2alias.o ... many thousands of lines omitted ...
If both the kernel and the modules compiles finished sucessfully, we're
ready to install them. The kernel image needs to be copied into a safe
location, typically the
/boot directory, and you
should give it a unique name to avoid overwriting any other kernel
images located there. Traditionaly kernel images are named
vmlinuz with the kernel release and local version
darkstar:/usr/src/linux# cat arch/x86/boot/bzImage > /boot/vmlinuz-release_number-local_version darkstar:/usr/src/linux# make modules_install
Once these steps have been completed, you will have a new kernel image
/boot and a new kernel modules
/lib/modules. In order to use
this new kernel, you will need to edit
create an initrd for it (only if you need to load one or more of this
kernel's modules to boot), and run lilo to
update the boot loader. When you reboot, if all went according to plan,
you should have an option to boot with your newly compiled kernel. If
something went wrong, you may be spending some time fixing the problem.
Previous Chapter: Keeping Track of Updates
- Original source: http://www.slackbook.org/beta
- Originally written by Alan Hicks, Chris Lumens, David Cantrell, Logan Johnson