view Documentation/filesystems/ramfs-rootfs-initramfs.txt @ 524:7f8b544237bf

netfront: Allow netfront in domain 0.

This is useful if your physical network device is in a utility domain.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
author Keir Fraser <keir.fraser@citrix.com>
date Tue Apr 15 15:18:58 2008 +0100 (2008-04-15)
parents 831230e53067
line source
1 ramfs, rootfs and initramfs
2 October 17, 2005
3 Rob Landley <rob@landley.net>
4 =============================
6 What is ramfs?
7 --------------
9 Ramfs is a very simple filesystem that exports Linux's disk caching
10 mechanisms (the page cache and dentry cache) as a dynamically resizable
11 ram-based filesystem.
13 Normally all files are cached in memory by Linux. Pages of data read from
14 backing store (usually the block device the filesystem is mounted on) are kept
15 around in case it's needed again, but marked as clean (freeable) in case the
16 Virtual Memory system needs the memory for something else. Similarly, data
17 written to files is marked clean as soon as it has been written to backing
18 store, but kept around for caching purposes until the VM reallocates the
19 memory. A similar mechanism (the dentry cache) greatly speeds up access to
20 directories.
22 With ramfs, there is no backing store. Files written into ramfs allocate
23 dentries and page cache as usual, but there's nowhere to write them to.
24 This means the pages are never marked clean, so they can't be freed by the
25 VM when it's looking to recycle memory.
27 The amount of code required to implement ramfs is tiny, because all the
28 work is done by the existing Linux caching infrastructure. Basically,
29 you're mounting the disk cache as a filesystem. Because of this, ramfs is not
30 an optional component removable via menuconfig, since there would be negligible
31 space savings.
33 ramfs and ramdisk:
34 ------------------
36 The older "ram disk" mechanism created a synthetic block device out of
37 an area of ram and used it as backing store for a filesystem. This block
38 device was of fixed size, so the filesystem mounted on it was of fixed
39 size. Using a ram disk also required unnecessarily copying memory from the
40 fake block device into the page cache (and copying changes back out), as well
41 as creating and destroying dentries. Plus it needed a filesystem driver
42 (such as ext2) to format and interpret this data.
44 Compared to ramfs, this wastes memory (and memory bus bandwidth), creates
45 unnecessary work for the CPU, and pollutes the CPU caches. (There are tricks
46 to avoid this copying by playing with the page tables, but they're unpleasantly
47 complicated and turn out to be about as expensive as the copying anyway.)
48 More to the point, all the work ramfs is doing has to happen _anyway_,
49 since all file access goes through the page and dentry caches. The ram
50 disk is simply unnecessary, ramfs is internally much simpler.
52 Another reason ramdisks are semi-obsolete is that the introduction of
53 loopback devices offered a more flexible and convenient way to create
54 synthetic block devices, now from files instead of from chunks of memory.
55 See losetup (8) for details.
57 ramfs and tmpfs:
58 ----------------
60 One downside of ramfs is you can keep writing data into it until you fill
61 up all memory, and the VM can't free it because the VM thinks that files
62 should get written to backing store (rather than swap space), but ramfs hasn't
63 got any backing store. Because of this, only root (or a trusted user) should
64 be allowed write access to a ramfs mount.
66 A ramfs derivative called tmpfs was created to add size limits, and the ability
67 to write the data to swap space. Normal users can be allowed write access to
68 tmpfs mounts. See Documentation/filesystems/tmpfs.txt for more information.
70 What is rootfs?
71 ---------------
73 Rootfs is a special instance of ramfs (or tmpfs, if that's enabled), which is
74 always present in 2.6 systems. You can't unmount rootfs for approximately the
75 same reason you can't kill the init process; rather than having special code
76 to check for and handle an empty list, it's smaller and simpler for the kernel
77 to just make sure certain lists can't become empty.
79 Most systems just mount another filesystem over rootfs and ignore it. The
80 amount of space an empty instance of ramfs takes up is tiny.
82 What is initramfs?
83 ------------------
85 All 2.6 Linux kernels contain a gzipped "cpio" format archive, which is
86 extracted into rootfs when the kernel boots up. After extracting, the kernel
87 checks to see if rootfs contains a file "init", and if so it executes it as PID
88 1. If found, this init process is responsible for bringing the system the
89 rest of the way up, including locating and mounting the real root device (if
90 any). If rootfs does not contain an init program after the embedded cpio
91 archive is extracted into it, the kernel will fall through to the older code
92 to locate and mount a root partition, then exec some variant of /sbin/init
93 out of that.
95 All this differs from the old initrd in several ways:
97 - The old initrd was always a separate file, while the initramfs archive is
98 linked into the linux kernel image. (The directory linux-*/usr is devoted
99 to generating this archive during the build.)
101 - The old initrd file was a gzipped filesystem image (in some file format,
102 such as ext2, that needed a driver built into the kernel), while the new
103 initramfs archive is a gzipped cpio archive (like tar only simpler,
104 see cpio(1) and Documentation/early-userspace/buffer-format.txt). The
105 kernel's cpio extraction code is not only extremely small, it's also
106 __init data that can be discarded during the boot process.
108 - The program run by the old initrd (which was called /initrd, not /init) did
109 some setup and then returned to the kernel, while the init program from
110 initramfs is not expected to return to the kernel. (If /init needs to hand
111 off control it can overmount / with a new root device and exec another init
112 program. See the switch_root utility, below.)
114 - When switching another root device, initrd would pivot_root and then
115 umount the ramdisk. But initramfs is rootfs: you can neither pivot_root
116 rootfs, nor unmount it. Instead delete everything out of rootfs to
117 free up the space (find -xdev / -exec rm '{}' ';'), overmount rootfs
118 with the new root (cd /newmount; mount --move . /; chroot .), attach
119 stdin/stdout/stderr to the new /dev/console, and exec the new init.
121 Since this is a remarkably persnickity process (and involves deleting
122 commands before you can run them), the klibc package introduced a helper
123 program (utils/run_init.c) to do all this for you. Most other packages
124 (such as busybox) have named this command "switch_root".
126 Populating initramfs:
127 ---------------------
129 The 2.6 kernel build process always creates a gzipped cpio format initramfs
130 archive and links it into the resulting kernel binary. By default, this
131 archive is empty (consuming 134 bytes on x86).
133 The config option CONFIG_INITRAMFS_SOURCE (for some reason buried under
134 devices->block devices in menuconfig, and living in usr/Kconfig) can be used
135 to specify a source for the initramfs archive, which will automatically be
136 incorporated into the resulting binary. This option can point to an existing
137 gzipped cpio archive, a directory containing files to be archived, or a text
138 file specification such as the following example:
140 dir /dev 755 0 0
141 nod /dev/console 644 0 0 c 5 1
142 nod /dev/loop0 644 0 0 b 7 0
143 dir /bin 755 1000 1000
144 slink /bin/sh busybox 777 0 0
145 file /bin/busybox initramfs/busybox 755 0 0
146 dir /proc 755 0 0
147 dir /sys 755 0 0
148 dir /mnt 755 0 0
149 file /init initramfs/init.sh 755 0 0
151 Run "usr/gen_init_cpio" (after the kernel build) to get a usage message
152 documenting the above file format.
154 One advantage of the configuration file is that root access is not required to
155 set permissions or create device nodes in the new archive. (Note that those
156 two example "file" entries expect to find files named "init.sh" and "busybox" in
157 a directory called "initramfs", under the linux-2.6.* directory. See
158 Documentation/early-userspace/README for more details.)
160 The kernel does not depend on external cpio tools. If you specify a
161 directory instead of a configuration file, the kernel's build infrastructure
162 creates a configuration file from that directory (usr/Makefile calls
163 scripts/gen_initramfs_list.sh), and proceeds to package up that directory
164 using the config file (by feeding it to usr/gen_init_cpio, which is created
165 from usr/gen_init_cpio.c). The kernel's build-time cpio creation code is
166 entirely self-contained, and the kernel's boot-time extractor is also
167 (obviously) self-contained.
169 The one thing you might need external cpio utilities installed for is creating
170 or extracting your own preprepared cpio files to feed to the kernel build
171 (instead of a config file or directory).
173 The following command line can extract a cpio image (either by the above script
174 or by the kernel build) back into its component files:
176 cpio -i -d -H newc -F initramfs_data.cpio --no-absolute-filenames
178 The following shell script can create a prebuilt cpio archive you can
179 use in place of the above config file:
181 #!/bin/sh
183 # Copyright 2006 Rob Landley <rob@landley.net> and TimeSys Corporation.
184 # Licensed under GPL version 2
186 if [ $# -ne 2 ]
187 then
188 echo "usage: mkinitramfs directory imagename.cpio.gz"
189 exit 1
190 fi
192 if [ -d "$1" ]
193 then
194 echo "creating $2 from $1"
195 (cd "$1"; find . | cpio -o -H newc | gzip) > "$2"
196 else
197 echo "First argument must be a directory"
198 exit 1
199 fi
201 Note: The cpio man page contains some bad advice that will break your initramfs
202 archive if you follow it. It says "A typical way to generate the list
203 of filenames is with the find command; you should give find the -depth option
204 to minimize problems with permissions on directories that are unwritable or not
205 searchable." Don't do this when creating initramfs.cpio.gz images, it won't
206 work. The Linux kernel cpio extractor won't create files in a directory that
207 doesn't exist, so the directory entries must go before the files that go in
208 those directories. The above script gets them in the right order.
210 External initramfs images:
211 --------------------------
213 If the kernel has initrd support enabled, an external cpio.gz archive can also
214 be passed into a 2.6 kernel in place of an initrd. In this case, the kernel
215 will autodetect the type (initramfs, not initrd) and extract the external cpio
216 archive into rootfs before trying to run /init.
218 This has the memory efficiency advantages of initramfs (no ramdisk block
219 device) but the separate packaging of initrd (which is nice if you have
220 non-GPL code you'd like to run from initramfs, without conflating it with
221 the GPL licensed Linux kernel binary).
223 It can also be used to supplement the kernel's built-in initamfs image. The
224 files in the external archive will overwrite any conflicting files in
225 the built-in initramfs archive. Some distributors also prefer to customize
226 a single kernel image with task-specific initramfs images, without recompiling.
228 Contents of initramfs:
229 ----------------------
231 An initramfs archive is a complete self-contained root filesystem for Linux.
232 If you don't already understand what shared libraries, devices, and paths
233 you need to get a minimal root filesystem up and running, here are some
234 references:
235 http://www.tldp.org/HOWTO/Bootdisk-HOWTO/
236 http://www.tldp.org/HOWTO/From-PowerUp-To-Bash-Prompt-HOWTO.html
237 http://www.linuxfromscratch.org/lfs/view/stable/
239 The "klibc" package (http://www.kernel.org/pub/linux/libs/klibc) is
240 designed to be a tiny C library to statically link early userspace
241 code against, along with some related utilities. It is BSD licensed.
243 I use uClibc (http://www.uclibc.org) and busybox (http://www.busybox.net)
244 myself. These are LGPL and GPL, respectively. (A self-contained initramfs
245 package is planned for the busybox 1.3 release.)
247 In theory you could use glibc, but that's not well suited for small embedded
248 uses like this. (A "hello world" program statically linked against glibc is
249 over 400k. With uClibc it's 7k. Also note that glibc dlopens libnss to do
250 name lookups, even when otherwise statically linked.)
252 A good first step is to get initramfs to run a statically linked "hello world"
253 program as init, and test it under an emulator like qemu (www.qemu.org) or
254 User Mode Linux, like so:
256 cat > hello.c << EOF
257 #include <stdio.h>
258 #include <unistd.h>
260 int main(int argc, char *argv[])
261 {
262 printf("Hello world!\n");
263 sleep(999999999);
264 }
265 EOF
266 gcc -static hello2.c -o init
267 echo init | cpio -o -H newc | gzip > test.cpio.gz
268 # Testing external initramfs using the initrd loading mechanism.
269 qemu -kernel /boot/vmlinuz -initrd test.cpio.gz /dev/zero
271 When debugging a normal root filesystem, it's nice to be able to boot with
272 "init=/bin/sh". The initramfs equivalent is "rdinit=/bin/sh", and it's
273 just as useful.
275 Why cpio rather than tar?
276 -------------------------
278 This decision was made back in December, 2001. The discussion started here:
280 http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1538.html
282 And spawned a second thread (specifically on tar vs cpio), starting here:
284 http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1587.html
286 The quick and dirty summary version (which is no substitute for reading
287 the above threads) is:
289 1) cpio is a standard. It's decades old (from the AT&T days), and already
290 widely used on Linux (inside RPM, Red Hat's device driver disks). Here's
291 a Linux Journal article about it from 1996:
293 http://www.linuxjournal.com/article/1213
295 It's not as popular as tar because the traditional cpio command line tools
296 require _truly_hideous_ command line arguments. But that says nothing
297 either way about the archive format, and there are alternative tools,
298 such as:
300 http://freshmeat.net/projects/afio/
302 2) The cpio archive format chosen by the kernel is simpler and cleaner (and
303 thus easier to create and parse) than any of the (literally dozens of)
304 various tar archive formats. The complete initramfs archive format is
305 explained in buffer-format.txt, created in usr/gen_init_cpio.c, and
306 extracted in init/initramfs.c. All three together come to less than 26k
307 total of human-readable text.
309 3) The GNU project standardizing on tar is approximately as relevant as
310 Windows standardizing on zip. Linux is not part of either, and is free
311 to make its own technical decisions.
313 4) Since this is a kernel internal format, it could easily have been
314 something brand new. The kernel provides its own tools to create and
315 extract this format anyway. Using an existing standard was preferable,
316 but not essential.
318 5) Al Viro made the decision (quote: "tar is ugly as hell and not going to be
319 supported on the kernel side"):
321 http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1540.html
323 explained his reasoning:
325 http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1550.html
326 http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1638.html
328 and, most importantly, designed and implemented the initramfs code.
330 Future directions:
331 ------------------
333 Today (2.6.16), initramfs is always compiled in, but not always used. The
334 kernel falls back to legacy boot code that is reached only if initramfs does
335 not contain an /init program. The fallback is legacy code, there to ensure a
336 smooth transition and allowing early boot functionality to gradually move to
337 "early userspace" (I.E. initramfs).
339 The move to early userspace is necessary because finding and mounting the real
340 root device is complex. Root partitions can span multiple devices (raid or
341 separate journal). They can be out on the network (requiring dhcp, setting a
342 specific mac address, logging into a server, etc). They can live on removable
343 media, with dynamically allocated major/minor numbers and persistent naming
344 issues requiring a full udev implementation to sort out. They can be
345 compressed, encrypted, copy-on-write, loopback mounted, strangely partitioned,
346 and so on.
348 This kind of complexity (which inevitably includes policy) is rightly handled
349 in userspace. Both klibc and busybox/uClibc are working on simple initramfs
350 packages to drop into a kernel build.
352 The klibc package has now been accepted into Andrew Morton's 2.6.17-mm tree.
353 The kernel's current early boot code (partition detection, etc) will probably
354 be migrated into a default initramfs, automatically created and used by the
355 kernel build.