initrd.txt

《嵌入式系统设计与实例开发实验教材二源码》Linux内核移植与编译实验

Using the initial RAM disk (initrd)
===================================

Written 1996,2000 by Werner Almesberger <[e-mail]> and
                     Hans Lermen <[e-mail]>


initrd provides the capability to load a RAM disk by the boot loader.
This RAM disk can then be mounted as the root file system and programs
can be run from it. Afterwards, a new root file system can be mounted
from a different device. The previous root (from initrd) is then moved
to a directory and can be subsequently unmounted.

initrd is mainly designed to allow system startup to occur in two phases,
where the kernel comes up with a minimum set of compiled-in drivers, and
where additional modules are loaded from initrd.

This document gives a brief overview of the use of initrd. A more detailed
discussion of the boot process can be found in [1].


Operation
---------

When using initrd, the system typically boots as follows:

  1) the boot loader loads the kernel and the initial RAM disk
  2) the kernel converts initrd into a "normal" RAM disk and
     frees the memory used by initrd
  3) initrd is mounted read-write as root
  4) /linuxrc is executed (this can be any valid executable, including
     shell scripts; it is run with uid 0 and can do basically everything
     init can do)
  5) linuxrc mounts the "real" root file system
  6) linuxrc places the root file system at the root directory using the
     pivot_root system call
  7) the usual boot sequence (e.g. invocation of /sbin/init) is performed
     on the root file system
  8) the initrd file system is removed

Note that changing the root directory does not involve unmounting it.
It is therefore possible to leave processes running on initrd during that
procedure. Also note that file systems mounted under initrd continue to
be accessible.


Boot command-line options
-------------------------

initrd adds the following new options:

  initrd=<path>    (e.g. LOADLIN)

    Loads the specified file as the initial RAM disk. When using LILO, you
    have to specify the RAM disk image file in /etc/lilo.conf, using the
    INITRD configuration variable.

  noinitrd

    initrd data is preserved but it is not converted to a RAM disk and
    the "normal" root file system is mounted. initrd data can be read
    from /dev/initrd. Note that the data in initrd can have any structure
    in this case and doesn't necessarily have to be a file system image.
    This option is used mainly for debugging.

    Note: /dev/initrd is read-only and it can only be used once. As soon
    as the last process has closed it, all data is freed and /dev/initrd
    can't be opened anymore.

  root=/dev/ram0   (without devfs)
  root=/dev/rd/0   (with devfs)

    initrd is mounted as root, and the normal boot procedure is followed,
    with the RAM disk still mounted as root.


Installation
------------

First, a directory for the initrd file system has to be created on the
"normal" root file system, e.g.

# mkdir /initrd

The name is not relevant. More details can be found on the pivot_root(2)
man page.

If the root file system is created during the boot procedure (i.e. if
you're building an install floppy), the root file system creation
procedure should create the /initrd directory.

If initrd will not be mounted in some cases, its content is still
accessible if the following device has been created (note that this
does not work if using devfs):

# mknod /dev/initrd b 1 250 
# chmod 400 /dev/initrd

Second, the kernel has to be compiled with RAM disk support and with
support for the initial RAM disk enabled. Also, at least all components
needed to execute programs from initrd (e.g. executable format and file
system) must be compiled into the kernel.

Third, you have to create the RAM disk image. This is done by creating a
file system on a block device, copying files to it as needed, and then
copying the content of the block device to the initrd file. With recent
kernels, at least three types of devices are suitable for that:

 - a floppy disk (works everywhere but it's painfully slow)
 - a RAM disk (fast, but allocates physical memory)
 - a loopback device (the most elegant solution)

We'll describe the loopback device method:

 1) make sure loopback block devices are configured into the kernel
 2) create an empty file system of the appropriate size, e.g.
    # dd if=/dev/zero of=initrd bs=300k count=1
    # mke2fs -F -m0 initrd
    (if space is critical, you may want to use the Minix FS instead of Ext2)
 3) mount the file system, e.g.
    # mount -t ext2 -o loop initrd /mnt
 4) create the console device (not necessary if using devfs, but it can't
    hurt to do it anyway):
    # mkdir /mnt/dev
    # mknod /mnt/dev/console c 5 1
 5) copy all the files that are needed to properly use the initrd
    environment. Don't forget the most important file, /linuxrc
    Note that /linuxrc's permissions must include "x" (execute).
 6) correct operation the initrd environment can frequently be tested
    even without rebooting with the command
    # chroot /mnt /linuxrc
    This is of course limited to initrds that do not interfere with the
    general system state (e.g. by reconfiguring network interfaces,
    overwriting mounted devices, trying to start already running demons,
    etc. Note however that it is usually possible to use pivot_root in
    such a chroot'ed initrd environment.)
 7) unmount the file system
    # umount /mnt
 8) the initrd is now in the file "initrd". Optionally, it can now be
    compressed
    # gzip -9 initrd

For experimenting with initrd, you may want to take a rescue floppy and
only add a symbolic link from /linuxrc to /bin/sh. Alternatively, you
can try the experimental newlib environment [2] to create a small
initrd.

Finally, you have to boot the kernel and load initrd. Almost all Linux
boot loaders support initrd. Since the boot process is still compatible
with an older mechanism, the following boot command line parameters
have to be given:

  root=/dev/ram0 init=/linuxrc rw

if not using devfs, or

  root=/dev/rd/0 init=/linuxrc rw

if using devfs. (rw is only necessary if writing to the initrd file
system.)

With LOADLIN, you simply execute

     LOADLIN <kernel> initrd=<disk_image>
e.g. LOADLIN C:\LINUX\BZIMAGE initrd=C:\LINUX\INITRD.GZ root=/dev/ram0
       init=/linuxrc rw

With LILO, you add the option INITRD=<path> to either the global section
or to the section of the respective kernel in /etc/lilo.conf, and pass
the options using APPEND, e.g.

  image = /bzImage
    initrd = /boot/initrd.gz
    append = "root=/dev/ram0 init=/linuxrc rw"

and run /sbin/lilo

For other boot loaders, please refer to the respective documentation.

Now you can boot and enjoy using initrd.


Changing the root device
------------------------

When finished with its duties, linuxrc typically changes the root device
and proceeds with starting the Linux system on the "real" root device.

The procedure involves the following steps:
 - mounting the new root file system
 - turning it into the root file system
 - removing all accesses to the old (initrd) root file system
 - unmounting the initrd file system and de-allocating the RAM disk

Mounting the new root file system is easy: it just needs to be mounted on
a directory under the current root. Example:

# mkdir /new-root
# mount -o ro /dev/hda1 /new-root

The root change is accomplished with the pivot_root system call, which
is also available via the pivot_root utility (see pivot_root(8) man
page; pivot_root is distributed with util-linux version 2.10h or higher
[3]). pivot_root moves the current root to a directory under the new
root, and puts the new root at its place. The directory for the old root
must exist before calling pivot_root. Example:

# cd /new-root
# mkdir initrd
# pivot_root . initrd

Now, the linuxrc process may still access the old root via its
executable, shared libraries, standard input/output/error, and its
current root directory. All these references are dropped by the
following command:

# exec chroot . what-follows <dev/console >dev/console 2>&1

Where what-follows is a program under the new root, e.g. /sbin/init
If the new root file system will be used with devfs and has no valid
/dev directory, devfs must be mounted before invoking chroot in order to
provide /dev/console.

Note: implementation details of pivot_root may change with time. In order
to ensure compatibility, the following points should be observed:

 - before calling pivot_root, the current directory of the invoking
   process should point to the new root directory
 - use . as the first argument, and the _relative_ path of the directory
   for the old root as the second argument
 - a chroot program must be available under the old and the new root
 - chroot to the new root afterwards
 - use relative paths for dev/console in the exec command

Now, the initrd can be unmounted and the memory allocated by the RAM
disk can be freed:

# umount /initrd
# blockdev --flushbufs /dev/ram0    # /dev/rd/0 if using devfs

It is also possible to use initrd with an NFS-mounted root, see the
pivot_root(8) man page for details.

Note: if linuxrc or any program exec'ed from it terminates for some
reason, the old change_root mechanism is invoked (see section "Obsolete
root change mechanism").


Usage scenarios
---------------

The main motivation for implementing initrd was to allow for modular
kernel configuration at system installation. The procedure would work
as follows:

  1) system boots from floppy or other media with a minimal kernel
     (e.g. support for RAM disks, initrd, a.out, and the Ext2 FS) and
     loads initrd
  2) /linuxrc determines what is needed to (1) mount the "real" root FS
     (i.e. device type, device drivers, file system) and (2) the
     distribution media (e.g. CD-ROM, network, tape, ...). This can be
     done by asking the user, by auto-probing, or by using a hybrid
     approach.
  3) /linuxrc loads the necessary kernel modules
  4) /linuxrc creates and populates the root file system (this doesn't
     have to be a very usable system yet)
  5) /linuxrc invokes pivot_root to change the root file system and
     execs - via chroot - a program that continues the installation
  6) the boot loader is installed
  7) the boot loader is configured to load an initrd with the set of
     modules that was used to bring up the system (e.g. /initrd can be
     modified, then unmounted, and finally, the image is written from
     /dev/ram0 or /dev/rd/0 to a file)
  8) now the system is bootable and additional installation tasks can be
     performed

The key role of initrd here is to re-use the configuration data during
normal system operation without requiring the use of a bloated "generic"
kernel or re-compiling or re-linking the kernel.

A second scenario is for installations where Linux runs on systems with
different hardware configurations in a single administrative domain. In
such cases, it is desirable to generate only a small set of kernels
(ideally only one) and to keep the system-specific part of configuration
information as small as possible. In this case, a common initrd could be
generated with all the necessary modules. Then, only /linuxrc or a file
read by it would have to be different.

A third scenario are more convenient recovery disks, because information
like the location of the root FS partition doesn't have to be provided at
boot time, but the system loaded from initrd can invoke a user-friendly
dialog and it can also perform some sanity checks (or even some form of
auto-detection).

Last not least, CD-ROM distributors may use it for better installation
from CD, e.g. by using a boot floppy and bootstrapping a bigger RAM disk
via initrd from CD; or by booting via a loader like LOADLIN or directly
from the CD-ROM, and loading the RAM disk from CD without need of
floppies. 


Obsolete root change mechanism
------------------------------

The following mechanism was used before the introduction of pivot_root.
Current kernels still support it, but you should _not_ rely on its
continued availability.

It works by mounting the "real" root device (i.e. the one set with rdev
in the kernel image or with root=... at the boot command line) as the
root file system when linuxrc exits. The initrd file system is then
unmounted, or, if it is still busy, moved to a directory /initrd, if
such a directory exists on the new root file system.

In order to use this mechanism, you do not have to specify the boot
command options root, init, or rw. (If specified, they will affect
the real root file system, not the initrd environment.)
  
If /proc is mounted, the "real" root device can be changed from within
linuxrc by writing the number of the new root FS device to the special
file /proc/sys/kernel/real-root-dev, e.g.

  # echo 0x301 >/proc/sys/kernel/real-root-dev

Note that the mechanism is incompatible with NFS and similar file
systems.

This old, deprecated mechanism is commonly called "change_root", while
the new, supported mechanism is called "pivot_root".


Resources
---------

[1] Almesberger, Werner; "Booting Linux: The History and the Future"
    ftp://icaftp.epfl.ch/pub/people/almesber/booting/bootinglinux-current.ps.gz
[2] newlib package (experimental), with initrd example
    ftp://icaftp.epfl.ch/pub/people/almesber/misc/newlib-linux/
[3] Brouwer, Andries; "util-linux: Miscellaneous utilities for Linux"
    ftp://ftp.win.tue.nl/pub/linux-local/utils/util-linux/