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diff --git a/Documentation/filesystems/ramfs-rootfs-initramfs.rst b/Documentation/filesystems/ramfs-rootfs-initramfs.rst new file mode 100644 index 000000000..164960631 --- /dev/null +++ b/Documentation/filesystems/ramfs-rootfs-initramfs.rst @@ -0,0 +1,369 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=========================== +Ramfs, rootfs and initramfs +=========================== + +October 17, 2005 + +Rob Landley <rob@landley.net> +============================= + +What is ramfs? +-------------- + +Ramfs is a very simple filesystem that exports Linux's disk caching +mechanisms (the page cache and dentry cache) as a dynamically resizable +RAM-based filesystem. + +Normally all files are cached in memory by Linux. Pages of data read from +backing store (usually the block device the filesystem is mounted on) are kept +around in case it's needed again, but marked as clean (freeable) in case the +Virtual Memory system needs the memory for something else. Similarly, data +written to files is marked clean as soon as it has been written to backing +store, but kept around for caching purposes until the VM reallocates the +memory. A similar mechanism (the dentry cache) greatly speeds up access to +directories. + +With ramfs, there is no backing store. Files written into ramfs allocate +dentries and page cache as usual, but there's nowhere to write them to. +This means the pages are never marked clean, so they can't be freed by the +VM when it's looking to recycle memory. + +The amount of code required to implement ramfs is tiny, because all the +work is done by the existing Linux caching infrastructure. Basically, +you're mounting the disk cache as a filesystem. Because of this, ramfs is not +an optional component removable via menuconfig, since there would be negligible +space savings. + +ramfs and ramdisk: +------------------ + +The older "ram disk" mechanism created a synthetic block device out of +an area of RAM and used it as backing store for a filesystem. This block +device was of fixed size, so the filesystem mounted on it was of fixed +size. Using a ram disk also required unnecessarily copying memory from the +fake block device into the page cache (and copying changes back out), as well +as creating and destroying dentries. Plus it needed a filesystem driver +(such as ext2) to format and interpret this data. + +Compared to ramfs, this wastes memory (and memory bus bandwidth), creates +unnecessary work for the CPU, and pollutes the CPU caches. (There are tricks +to avoid this copying by playing with the page tables, but they're unpleasantly +complicated and turn out to be about as expensive as the copying anyway.) +More to the point, all the work ramfs is doing has to happen _anyway_, +since all file access goes through the page and dentry caches. The RAM +disk is simply unnecessary; ramfs is internally much simpler. + +Another reason ramdisks are semi-obsolete is that the introduction of +loopback devices offered a more flexible and convenient way to create +synthetic block devices, now from files instead of from chunks of memory. +See losetup (8) for details. + +ramfs and tmpfs: +---------------- + +One downside of ramfs is you can keep writing data into it until you fill +up all memory, and the VM can't free it because the VM thinks that files +should get written to backing store (rather than swap space), but ramfs hasn't +got any backing store. Because of this, only root (or a trusted user) should +be allowed write access to a ramfs mount. + +A ramfs derivative called tmpfs was created to add size limits, and the ability +to write the data to swap space. Normal users can be allowed write access to +tmpfs mounts. See Documentation/filesystems/tmpfs.rst for more information. + +What is rootfs? +--------------- + +Rootfs is a special instance of ramfs (or tmpfs, if that's enabled), which is +always present in 2.6 systems. You can't unmount rootfs for approximately the +same reason you can't kill the init process; rather than having special code +to check for and handle an empty list, it's smaller and simpler for the kernel +to just make sure certain lists can't become empty. + +Most systems just mount another filesystem over rootfs and ignore it. The +amount of space an empty instance of ramfs takes up is tiny. + +If CONFIG_TMPFS is enabled, rootfs will use tmpfs instead of ramfs by +default. To force ramfs, add "rootfstype=ramfs" to the kernel command +line. + +What is initramfs? +------------------ + +All 2.6 Linux kernels contain a gzipped "cpio" format archive, which is +extracted into rootfs when the kernel boots up. After extracting, the kernel +checks to see if rootfs contains a file "init", and if so it executes it as PID +1. If found, this init process is responsible for bringing the system the +rest of the way up, including locating and mounting the real root device (if +any). If rootfs does not contain an init program after the embedded cpio +archive is extracted into it, the kernel will fall through to the older code +to locate and mount a root partition, then exec some variant of /sbin/init +out of that. + +All this differs from the old initrd in several ways: + + - The old initrd was always a separate file, while the initramfs archive is + linked into the linux kernel image. (The directory ``linux-*/usr`` is + devoted to generating this archive during the build.) + + - The old initrd file was a gzipped filesystem image (in some file format, + such as ext2, that needed a driver built into the kernel), while the new + initramfs archive is a gzipped cpio archive (like tar only simpler, + see cpio(1) and Documentation/driver-api/early-userspace/buffer-format.rst). + The kernel's cpio extraction code is not only extremely small, it's also + __init text and data that can be discarded during the boot process. + + - The program run by the old initrd (which was called /initrd, not /init) did + some setup and then returned to the kernel, while the init program from + initramfs is not expected to return to the kernel. (If /init needs to hand + off control it can overmount / with a new root device and exec another init + program. See the switch_root utility, below.) + + - When switching another root device, initrd would pivot_root and then + umount the ramdisk. But initramfs is rootfs: you can neither pivot_root + rootfs, nor unmount it. Instead delete everything out of rootfs to + free up the space (find -xdev / -exec rm '{}' ';'), overmount rootfs + with the new root (cd /newmount; mount --move . /; chroot .), attach + stdin/stdout/stderr to the new /dev/console, and exec the new init. + + Since this is a remarkably persnickety process (and involves deleting + commands before you can run them), the klibc package introduced a helper + program (utils/run_init.c) to do all this for you. Most other packages + (such as busybox) have named this command "switch_root". + +Populating initramfs: +--------------------- + +The 2.6 kernel build process always creates a gzipped cpio format initramfs +archive and links it into the resulting kernel binary. By default, this +archive is empty (consuming 134 bytes on x86). + +The config option CONFIG_INITRAMFS_SOURCE (in General Setup in menuconfig, +and living in usr/Kconfig) can be used to specify a source for the +initramfs archive, which will automatically be incorporated into the +resulting binary. This option can point to an existing gzipped cpio +archive, a directory containing files to be archived, or a text file +specification such as the following example:: + + dir /dev 755 0 0 + nod /dev/console 644 0 0 c 5 1 + nod /dev/loop0 644 0 0 b 7 0 + dir /bin 755 1000 1000 + slink /bin/sh busybox 777 0 0 + file /bin/busybox initramfs/busybox 755 0 0 + dir /proc 755 0 0 + dir /sys 755 0 0 + dir /mnt 755 0 0 + file /init initramfs/init.sh 755 0 0 + +Run "usr/gen_init_cpio" (after the kernel build) to get a usage message +documenting the above file format. + +One advantage of the configuration file is that root access is not required to +set permissions or create device nodes in the new archive. (Note that those +two example "file" entries expect to find files named "init.sh" and "busybox" in +a directory called "initramfs", under the linux-2.6.* directory. See +Documentation/driver-api/early-userspace/early_userspace_support.rst for more details.) + +The kernel does not depend on external cpio tools. If you specify a +directory instead of a configuration file, the kernel's build infrastructure +creates a configuration file from that directory (usr/Makefile calls +usr/gen_initramfs.sh), and proceeds to package up that directory +using the config file (by feeding it to usr/gen_init_cpio, which is created +from usr/gen_init_cpio.c). The kernel's build-time cpio creation code is +entirely self-contained, and the kernel's boot-time extractor is also +(obviously) self-contained. + +The one thing you might need external cpio utilities installed for is creating +or extracting your own preprepared cpio files to feed to the kernel build +(instead of a config file or directory). + +The following command line can extract a cpio image (either by the above script +or by the kernel build) back into its component files:: + + cpio -i -d -H newc -F initramfs_data.cpio --no-absolute-filenames + +The following shell script can create a prebuilt cpio archive you can +use in place of the above config file:: + + #!/bin/sh + + # Copyright 2006 Rob Landley <rob@landley.net> and TimeSys Corporation. + # Licensed under GPL version 2 + + if [ $# -ne 2 ] + then + echo "usage: mkinitramfs directory imagename.cpio.gz" + exit 1 + fi + + if [ -d "$1" ] + then + echo "creating $2 from $1" + (cd "$1"; find . | cpio -o -H newc | gzip) > "$2" + else + echo "First argument must be a directory" + exit 1 + fi + +.. Note:: + + The cpio man page contains some bad advice that will break your initramfs + archive if you follow it. It says "A typical way to generate the list + of filenames is with the find command; you should give find the -depth + option to minimize problems with permissions on directories that are + unwritable or not searchable." Don't do this when creating + initramfs.cpio.gz images, it won't work. The Linux kernel cpio extractor + won't create files in a directory that doesn't exist, so the directory + entries must go before the files that go in those directories. + The above script gets them in the right order. + +External initramfs images: +-------------------------- + +If the kernel has initrd support enabled, an external cpio.gz archive can also +be passed into a 2.6 kernel in place of an initrd. In this case, the kernel +will autodetect the type (initramfs, not initrd) and extract the external cpio +archive into rootfs before trying to run /init. + +This has the memory efficiency advantages of initramfs (no ramdisk block +device) but the separate packaging of initrd (which is nice if you have +non-GPL code you'd like to run from initramfs, without conflating it with +the GPL licensed Linux kernel binary). + +It can also be used to supplement the kernel's built-in initramfs image. The +files in the external archive will overwrite any conflicting files in +the built-in initramfs archive. Some distributors also prefer to customize +a single kernel image with task-specific initramfs images, without recompiling. + +Contents of initramfs: +---------------------- + +An initramfs archive is a complete self-contained root filesystem for Linux. +If you don't already understand what shared libraries, devices, and paths +you need to get a minimal root filesystem up and running, here are some +references: + +- https://www.tldp.org/HOWTO/Bootdisk-HOWTO/ +- https://www.tldp.org/HOWTO/From-PowerUp-To-Bash-Prompt-HOWTO.html +- http://www.linuxfromscratch.org/lfs/view/stable/ + +The "klibc" package (https://www.kernel.org/pub/linux/libs/klibc) is +designed to be a tiny C library to statically link early userspace +code against, along with some related utilities. It is BSD licensed. + +I use uClibc (https://www.uclibc.org) and busybox (https://www.busybox.net) +myself. These are LGPL and GPL, respectively. (A self-contained initramfs +package is planned for the busybox 1.3 release.) + +In theory you could use glibc, but that's not well suited for small embedded +uses like this. (A "hello world" program statically linked against glibc is +over 400k. With uClibc it's 7k. Also note that glibc dlopens libnss to do +name lookups, even when otherwise statically linked.) + +A good first step is to get initramfs to run a statically linked "hello world" +program as init, and test it under an emulator like qemu (www.qemu.org) or +User Mode Linux, like so:: + + cat > hello.c << EOF + #include <stdio.h> + #include <unistd.h> + + int main(int argc, char *argv[]) + { + printf("Hello world!\n"); + sleep(999999999); + } + EOF + gcc -static hello.c -o init + echo init | cpio -o -H newc | gzip > test.cpio.gz + # Testing external initramfs using the initrd loading mechanism. + qemu -kernel /boot/vmlinuz -initrd test.cpio.gz /dev/zero + +When debugging a normal root filesystem, it's nice to be able to boot with +"init=/bin/sh". The initramfs equivalent is "rdinit=/bin/sh", and it's +just as useful. + +Why cpio rather than tar? +------------------------- + +This decision was made back in December, 2001. The discussion started here: + + http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1538.html + +And spawned a second thread (specifically on tar vs cpio), starting here: + + http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1587.html + +The quick and dirty summary version (which is no substitute for reading +the above threads) is: + +1) cpio is a standard. It's decades old (from the AT&T days), and already + widely used on Linux (inside RPM, Red Hat's device driver disks). Here's + a Linux Journal article about it from 1996: + + http://www.linuxjournal.com/article/1213 + + It's not as popular as tar because the traditional cpio command line tools + require _truly_hideous_ command line arguments. But that says nothing + either way about the archive format, and there are alternative tools, + such as: + + http://freecode.com/projects/afio + +2) The cpio archive format chosen by the kernel is simpler and cleaner (and + thus easier to create and parse) than any of the (literally dozens of) + various tar archive formats. The complete initramfs archive format is + explained in buffer-format.txt, created in usr/gen_init_cpio.c, and + extracted in init/initramfs.c. All three together come to less than 26k + total of human-readable text. + +3) The GNU project standardizing on tar is approximately as relevant as + Windows standardizing on zip. Linux is not part of either, and is free + to make its own technical decisions. + +4) Since this is a kernel internal format, it could easily have been + something brand new. The kernel provides its own tools to create and + extract this format anyway. Using an existing standard was preferable, + but not essential. + +5) Al Viro made the decision (quote: "tar is ugly as hell and not going to be + supported on the kernel side"): + + http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1540.html + + explained his reasoning: + + - http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1550.html + - http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1638.html + + and, most importantly, designed and implemented the initramfs code. + +Future directions: +------------------ + +Today (2.6.16), initramfs is always compiled in, but not always used. The +kernel falls back to legacy boot code that is reached only if initramfs does +not contain an /init program. The fallback is legacy code, there to ensure a +smooth transition and allowing early boot functionality to gradually move to +"early userspace" (I.E. initramfs). + +The move to early userspace is necessary because finding and mounting the real +root device is complex. Root partitions can span multiple devices (raid or +separate journal). They can be out on the network (requiring dhcp, setting a +specific MAC address, logging into a server, etc). They can live on removable +media, with dynamically allocated major/minor numbers and persistent naming +issues requiring a full udev implementation to sort out. They can be +compressed, encrypted, copy-on-write, loopback mounted, strangely partitioned, +and so on. + +This kind of complexity (which inevitably includes policy) is rightly handled +in userspace. Both klibc and busybox/uClibc are working on simple initramfs +packages to drop into a kernel build. + +The klibc package has now been accepted into Andrew Morton's 2.6.17-mm tree. +The kernel's current early boot code (partition detection, etc) will probably +be migrated into a default initramfs, automatically created and used by the +kernel build. |