Adding upstream version 5.10.209.upstream/5.10.209upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+.. 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.