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.txt 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/early-userspace/buffer-format.txt).  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/early-userspace/README 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
  scripts/gen_initramfs_list.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:
  http://www.tldp.org/HOWTO/Bootdisk-HOWTO/
  http://www.tldp.org/HOWTO/From-PowerUp-To-Bash-Prompt-HOWTO.html
  http://www.linuxfromscratch.org/lfs/view/stable/
  
  The "klibc" package (http://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 (http://www.uclibc.org) and busybox (http://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!
  ");
      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.