view Documentation/initrd.txt @ 452:c7ed6fe5dca0

kexec: dont initialise regions in reserve_memory()

There is no need to initialise efi_memmap_res and boot_param_res in
reserve_memory() for the initial xen domain as it is done in
machine_kexec_setup_resources() using values from the kexec hypercall.

Signed-off-by: Simon Horman <horms@verge.net.au>
author Keir Fraser <keir.fraser@citrix.com>
date Thu Feb 28 10:55:18 2008 +0000 (2008-02-28)
parents 831230e53067
line source
1 Using the initial RAM disk (initrd)
2 ===================================
4 Written 1996,2000 by Werner Almesberger <werner.almesberger@epfl.ch> and
5 Hans Lermen <lermen@fgan.de>
8 initrd provides the capability to load a RAM disk by the boot loader.
9 This RAM disk can then be mounted as the root file system and programs
10 can be run from it. Afterwards, a new root file system can be mounted
11 from a different device. The previous root (from initrd) is then moved
12 to a directory and can be subsequently unmounted.
14 initrd is mainly designed to allow system startup to occur in two phases,
15 where the kernel comes up with a minimum set of compiled-in drivers, and
16 where additional modules are loaded from initrd.
18 This document gives a brief overview of the use of initrd. A more detailed
19 discussion of the boot process can be found in [1].
22 Operation
23 ---------
25 When using initrd, the system typically boots as follows:
27 1) the boot loader loads the kernel and the initial RAM disk
28 2) the kernel converts initrd into a "normal" RAM disk and
29 frees the memory used by initrd
30 3) initrd is mounted read-write as root
31 4) /linuxrc is executed (this can be any valid executable, including
32 shell scripts; it is run with uid 0 and can do basically everything
33 init can do)
34 5) linuxrc mounts the "real" root file system
35 6) linuxrc places the root file system at the root directory using the
36 pivot_root system call
37 7) the usual boot sequence (e.g. invocation of /sbin/init) is performed
38 on the root file system
39 8) the initrd file system is removed
41 Note that changing the root directory does not involve unmounting it.
42 It is therefore possible to leave processes running on initrd during that
43 procedure. Also note that file systems mounted under initrd continue to
44 be accessible.
47 Boot command-line options
48 -------------------------
50 initrd adds the following new options:
52 initrd=<path> (e.g. LOADLIN)
54 Loads the specified file as the initial RAM disk. When using LILO, you
55 have to specify the RAM disk image file in /etc/lilo.conf, using the
56 INITRD configuration variable.
58 noinitrd
60 initrd data is preserved but it is not converted to a RAM disk and
61 the "normal" root file system is mounted. initrd data can be read
62 from /dev/initrd. Note that the data in initrd can have any structure
63 in this case and doesn't necessarily have to be a file system image.
64 This option is used mainly for debugging.
66 Note: /dev/initrd is read-only and it can only be used once. As soon
67 as the last process has closed it, all data is freed and /dev/initrd
68 can't be opened anymore.
70 root=/dev/ram0
72 initrd is mounted as root, and the normal boot procedure is followed,
73 with the RAM disk still mounted as root.
75 Compressed cpio images
76 ----------------------
78 Recent kernels have support for populating a ramdisk from a compressed cpio
79 archive, on such systems, the creation of a ramdisk image doesn't need to
80 involve special block devices or loopbacks, you merely create a directory on
81 disk with the desired initrd content, cd to that directory, and run (as an
82 example):
84 find . | cpio --quiet -c -o | gzip -9 -n > /boot/imagefile.img
86 Examining the contents of an existing image file is just as simple:
88 mkdir /tmp/imagefile
89 cd /tmp/imagefile
90 gzip -cd /boot/imagefile.img | cpio -imd --quiet
92 Installation
93 ------------
95 First, a directory for the initrd file system has to be created on the
96 "normal" root file system, e.g.
98 # mkdir /initrd
100 The name is not relevant. More details can be found on the pivot_root(2)
101 man page.
103 If the root file system is created during the boot procedure (i.e. if
104 you're building an install floppy), the root file system creation
105 procedure should create the /initrd directory.
107 If initrd will not be mounted in some cases, its content is still
108 accessible if the following device has been created:
110 # mknod /dev/initrd b 1 250
111 # chmod 400 /dev/initrd
113 Second, the kernel has to be compiled with RAM disk support and with
114 support for the initial RAM disk enabled. Also, at least all components
115 needed to execute programs from initrd (e.g. executable format and file
116 system) must be compiled into the kernel.
118 Third, you have to create the RAM disk image. This is done by creating a
119 file system on a block device, copying files to it as needed, and then
120 copying the content of the block device to the initrd file. With recent
121 kernels, at least three types of devices are suitable for that:
123 - a floppy disk (works everywhere but it's painfully slow)
124 - a RAM disk (fast, but allocates physical memory)
125 - a loopback device (the most elegant solution)
127 We'll describe the loopback device method:
129 1) make sure loopback block devices are configured into the kernel
130 2) create an empty file system of the appropriate size, e.g.
131 # dd if=/dev/zero of=initrd bs=300k count=1
132 # mke2fs -F -m0 initrd
133 (if space is critical, you may want to use the Minix FS instead of Ext2)
134 3) mount the file system, e.g.
135 # mount -t ext2 -o loop initrd /mnt
136 4) create the console device:
137 # mkdir /mnt/dev
138 # mknod /mnt/dev/console c 5 1
139 5) copy all the files that are needed to properly use the initrd
140 environment. Don't forget the most important file, /linuxrc
141 Note that /linuxrc's permissions must include "x" (execute).
142 6) correct operation the initrd environment can frequently be tested
143 even without rebooting with the command
144 # chroot /mnt /linuxrc
145 This is of course limited to initrds that do not interfere with the
146 general system state (e.g. by reconfiguring network interfaces,
147 overwriting mounted devices, trying to start already running demons,
148 etc. Note however that it is usually possible to use pivot_root in
149 such a chroot'ed initrd environment.)
150 7) unmount the file system
151 # umount /mnt
152 8) the initrd is now in the file "initrd". Optionally, it can now be
153 compressed
154 # gzip -9 initrd
156 For experimenting with initrd, you may want to take a rescue floppy and
157 only add a symbolic link from /linuxrc to /bin/sh. Alternatively, you
158 can try the experimental newlib environment [2] to create a small
159 initrd.
161 Finally, you have to boot the kernel and load initrd. Almost all Linux
162 boot loaders support initrd. Since the boot process is still compatible
163 with an older mechanism, the following boot command line parameters
164 have to be given:
166 root=/dev/ram0 init=/linuxrc rw
168 (rw is only necessary if writing to the initrd file system.)
170 With LOADLIN, you simply execute
172 LOADLIN <kernel> initrd=<disk_image>
173 e.g. LOADLIN C:\LINUX\BZIMAGE initrd=C:\LINUX\INITRD.GZ root=/dev/ram0
174 init=/linuxrc rw
176 With LILO, you add the option INITRD=<path> to either the global section
177 or to the section of the respective kernel in /etc/lilo.conf, and pass
178 the options using APPEND, e.g.
180 image = /bzImage
181 initrd = /boot/initrd.gz
182 append = "root=/dev/ram0 init=/linuxrc rw"
184 and run /sbin/lilo
186 For other boot loaders, please refer to the respective documentation.
188 Now you can boot and enjoy using initrd.
191 Changing the root device
192 ------------------------
194 When finished with its duties, linuxrc typically changes the root device
195 and proceeds with starting the Linux system on the "real" root device.
197 The procedure involves the following steps:
198 - mounting the new root file system
199 - turning it into the root file system
200 - removing all accesses to the old (initrd) root file system
201 - unmounting the initrd file system and de-allocating the RAM disk
203 Mounting the new root file system is easy: it just needs to be mounted on
204 a directory under the current root. Example:
206 # mkdir /new-root
207 # mount -o ro /dev/hda1 /new-root
209 The root change is accomplished with the pivot_root system call, which
210 is also available via the pivot_root utility (see pivot_root(8) man
211 page; pivot_root is distributed with util-linux version 2.10h or higher
212 [3]). pivot_root moves the current root to a directory under the new
213 root, and puts the new root at its place. The directory for the old root
214 must exist before calling pivot_root. Example:
216 # cd /new-root
217 # mkdir initrd
218 # pivot_root . initrd
220 Now, the linuxrc process may still access the old root via its
221 executable, shared libraries, standard input/output/error, and its
222 current root directory. All these references are dropped by the
223 following command:
225 # exec chroot . what-follows <dev/console >dev/console 2>&1
227 Where what-follows is a program under the new root, e.g. /sbin/init
228 If the new root file system will be used with udev and has no valid
229 /dev directory, udev must be initialized before invoking chroot in order
230 to provide /dev/console.
232 Note: implementation details of pivot_root may change with time. In order
233 to ensure compatibility, the following points should be observed:
235 - before calling pivot_root, the current directory of the invoking
236 process should point to the new root directory
237 - use . as the first argument, and the _relative_ path of the directory
238 for the old root as the second argument
239 - a chroot program must be available under the old and the new root
240 - chroot to the new root afterwards
241 - use relative paths for dev/console in the exec command
243 Now, the initrd can be unmounted and the memory allocated by the RAM
244 disk can be freed:
246 # umount /initrd
247 # blockdev --flushbufs /dev/ram0
249 It is also possible to use initrd with an NFS-mounted root, see the
250 pivot_root(8) man page for details.
252 Note: if linuxrc or any program exec'ed from it terminates for some
253 reason, the old change_root mechanism is invoked (see section "Obsolete
254 root change mechanism").
257 Usage scenarios
258 ---------------
260 The main motivation for implementing initrd was to allow for modular
261 kernel configuration at system installation. The procedure would work
262 as follows:
264 1) system boots from floppy or other media with a minimal kernel
265 (e.g. support for RAM disks, initrd, a.out, and the Ext2 FS) and
266 loads initrd
267 2) /linuxrc determines what is needed to (1) mount the "real" root FS
268 (i.e. device type, device drivers, file system) and (2) the
269 distribution media (e.g. CD-ROM, network, tape, ...). This can be
270 done by asking the user, by auto-probing, or by using a hybrid
271 approach.
272 3) /linuxrc loads the necessary kernel modules
273 4) /linuxrc creates and populates the root file system (this doesn't
274 have to be a very usable system yet)
275 5) /linuxrc invokes pivot_root to change the root file system and
276 execs - via chroot - a program that continues the installation
277 6) the boot loader is installed
278 7) the boot loader is configured to load an initrd with the set of
279 modules that was used to bring up the system (e.g. /initrd can be
280 modified, then unmounted, and finally, the image is written from
281 /dev/ram0 or /dev/rd/0 to a file)
282 8) now the system is bootable and additional installation tasks can be
283 performed
285 The key role of initrd here is to re-use the configuration data during
286 normal system operation without requiring the use of a bloated "generic"
287 kernel or re-compiling or re-linking the kernel.
289 A second scenario is for installations where Linux runs on systems with
290 different hardware configurations in a single administrative domain. In
291 such cases, it is desirable to generate only a small set of kernels
292 (ideally only one) and to keep the system-specific part of configuration
293 information as small as possible. In this case, a common initrd could be
294 generated with all the necessary modules. Then, only /linuxrc or a file
295 read by it would have to be different.
297 A third scenario are more convenient recovery disks, because information
298 like the location of the root FS partition doesn't have to be provided at
299 boot time, but the system loaded from initrd can invoke a user-friendly
300 dialog and it can also perform some sanity checks (or even some form of
301 auto-detection).
303 Last not least, CD-ROM distributors may use it for better installation
304 from CD, e.g. by using a boot floppy and bootstrapping a bigger RAM disk
305 via initrd from CD; or by booting via a loader like LOADLIN or directly
306 from the CD-ROM, and loading the RAM disk from CD without need of
307 floppies.
310 Obsolete root change mechanism
311 ------------------------------
313 The following mechanism was used before the introduction of pivot_root.
314 Current kernels still support it, but you should _not_ rely on its
315 continued availability.
317 It works by mounting the "real" root device (i.e. the one set with rdev
318 in the kernel image or with root=... at the boot command line) as the
319 root file system when linuxrc exits. The initrd file system is then
320 unmounted, or, if it is still busy, moved to a directory /initrd, if
321 such a directory exists on the new root file system.
323 In order to use this mechanism, you do not have to specify the boot
324 command options root, init, or rw. (If specified, they will affect
325 the real root file system, not the initrd environment.)
327 If /proc is mounted, the "real" root device can be changed from within
328 linuxrc by writing the number of the new root FS device to the special
329 file /proc/sys/kernel/real-root-dev, e.g.
331 # echo 0x301 >/proc/sys/kernel/real-root-dev
333 Note that the mechanism is incompatible with NFS and similar file
334 systems.
336 This old, deprecated mechanism is commonly called "change_root", while
337 the new, supported mechanism is called "pivot_root".
340 Resources
341 ---------
343 [1] Almesberger, Werner; "Booting Linux: The History and the Future"
344 http://www.almesberger.net/cv/papers/ols2k-9.ps.gz
345 [2] newlib package (experimental), with initrd example
346 http://sources.redhat.com/newlib/
347 [3] Brouwer, Andries; "util-linux: Miscellaneous utilities for Linux"
348 ftp://ftp.win.tue.nl/pub/linux-local/utils/util-linux/