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 ===========================
 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.

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