Documentation / filesystems / ramfs-rootfs-initramfs.txt

http://www.mjmwired.net/kernel/Documentation/filesystems/ramfs-rootfs-initramfs.txt

 1 ramfs, rootfs and initramfs

2	October 17, 2005
3	Rob Landley <rob[AT]landley[DOT]net>
4	=============================
5	
6	What is ramfs?
7	--------------
8	
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.
12	
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.
21	
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.
26	
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.
32	
33	ramfs and ramdisk:
34	------------------
35	
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.
43	
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.
51	
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.
56	
57	ramfs and tmpfs:
58	----------------
59	
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.
65	
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.
69	
70	What is rootfs?
71	---------------
72	
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.
78	
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.
81	
82	What is initramfs?
83	------------------
84	
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.
94	
95	All this differs from the old initrd in several ways:
96	
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.)
100	
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 text and data that can be discarded during the boot process.
107	
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.)
113	
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.
120	
121	    Since this is a remarkably persnickety 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".
125	
126	Populating initramfs:
127	---------------------
128	
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).
132	
133	The config option CONFIG_INITRAMFS_SOURCE (in General Setup in menuconfig,
134	and living in usr/Kconfig) can be used to specify a source for the
135	initramfs archive, which will automatically be incorporated into the
136	resulting binary.  This option can point to an existing gzipped cpio
137	archive, a directory containing files to be archived, or a text file
138	specification such as the following example:
139	
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
150	
151	Run "usr/gen_init_cpio" (after the kernel build) to get a usage message
152	documenting the above file format.
153	
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.)
159	
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.
168	
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).
172	
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:
175	
176	  cpio -i -d -H newc -F initramfs_data.cpio --no-absolute-filenames
177	
178	The following shell script can create a prebuilt cpio archive you can
179	use in place of the above config file:
180	
181	  #!/bin/sh
182	
183	  # Copyright 2006 Rob Landley <rob[AT]landley.net> and TimeSys Corporation[DOT]
184	  # Licensed under GPL version 2
185	
186	  if [ $# -ne 2 ]
187	  then
188	    echo "usage: mkinitramfs directory imagename.cpio.gz"
189	    exit 1
190	  fi
191	
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
200	
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.
209	
210	External initramfs images:
211	--------------------------
212	
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.
217	
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).
222	
223	It can also be used to supplement the kernel's built-in initramfs 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.
227	
228	Contents of initramfs:
229	----------------------
230	
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/
238	
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.
242	
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.)
246	
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.)
251	
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:
255	
256	  cat > hello.c << EOF
257	  #include <stdio.h>
258	  #include <unistd.h>
259	
260	  int main(int argc, char *argv[])
261	  {
262	    printf("Hello world!/n");
263	    sleep(999999999);
264	  }
265	  EOF
266	  gcc -static hello.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
270	
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.
274	
275	Why cpio rather than tar?
276	-------------------------
277	
278	This decision was made back in December, 2001.  The discussion started here:
279	
280	  http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1538.html
281	
282	And spawned a second thread (specifically on tar vs cpio), starting here:
283	
284	  http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1587.html
285	
286	The quick and dirty summary version (which is no substitute for reading
287	the above threads) is:
288	
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:
292	
293	      http://www.linuxjournal.com/article/1213
294	
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:
299	
300	     http://freshmeat.net/projects/afio/
301	
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.
308	
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.
312	
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.
317	
318	5) Al Viro made the decision (quote: "tar is ugly as hell and not going to be
319	   supported on the kernel side"):
320	
321	      http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1540.html
322	
323	   explained his reasoning:
324	
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
327	
328	   and, most importantly, designed and implemented the initramfs code.
329	
330	Future directions:
331	------------------
332	
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).
338	
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.
347	
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.
351	
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