![Rob Landley](/assets/img/avatar_default.png)
New section on creating an external initramfs image using cpio (with script), a warning about bad advice in the cpio man page, a bit of debugging advice (hello world and rdinit=/bin/sh), and a few minor tweaks to other parts of it. Signed-off-by: Rob Landley <rob@landley.net> Cc: "H. Peter Anvin" <hpa@zytor.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
356 lines
16 KiB
Text
356 lines
16 KiB
Text
ramfs, rootfs and initramfs
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October 17, 2005
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Rob Landley <rob@landley.net>
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=============================
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What is ramfs?
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--------------
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Ramfs is a very simple filesystem that exports Linux's disk caching
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mechanisms (the page cache and dentry cache) as a dynamically resizable
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ram-based filesystem.
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Normally all files are cached in memory by Linux. Pages of data read from
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backing store (usually the block device the filesystem is mounted on) are kept
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around in case it's needed again, but marked as clean (freeable) in case the
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Virtual Memory system needs the memory for something else. Similarly, data
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written to files is marked clean as soon as it has been written to backing
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store, but kept around for caching purposes until the VM reallocates the
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memory. A similar mechanism (the dentry cache) greatly speeds up access to
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directories.
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With ramfs, there is no backing store. Files written into ramfs allocate
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dentries and page cache as usual, but there's nowhere to write them to.
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This means the pages are never marked clean, so they can't be freed by the
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VM when it's looking to recycle memory.
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The amount of code required to implement ramfs is tiny, because all the
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work is done by the existing Linux caching infrastructure. Basically,
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you're mounting the disk cache as a filesystem. Because of this, ramfs is not
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an optional component removable via menuconfig, since there would be negligible
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space savings.
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ramfs and ramdisk:
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------------------
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The older "ram disk" mechanism created a synthetic block device out of
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an area of ram and used it as backing store for a filesystem. This block
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device was of fixed size, so the filesystem mounted on it was of fixed
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size. Using a ram disk also required unnecessarily copying memory from the
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fake block device into the page cache (and copying changes back out), as well
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as creating and destroying dentries. Plus it needed a filesystem driver
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(such as ext2) to format and interpret this data.
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Compared to ramfs, this wastes memory (and memory bus bandwidth), creates
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unnecessary work for the CPU, and pollutes the CPU caches. (There are tricks
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to avoid this copying by playing with the page tables, but they're unpleasantly
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complicated and turn out to be about as expensive as the copying anyway.)
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More to the point, all the work ramfs is doing has to happen _anyway_,
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since all file access goes through the page and dentry caches. The ram
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disk is simply unnecessary, ramfs is internally much simpler.
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Another reason ramdisks are semi-obsolete is that the introduction of
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loopback devices offered a more flexible and convenient way to create
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synthetic block devices, now from files instead of from chunks of memory.
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See losetup (8) for details.
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ramfs and tmpfs:
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----------------
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One downside of ramfs is you can keep writing data into it until you fill
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up all memory, and the VM can't free it because the VM thinks that files
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should get written to backing store (rather than swap space), but ramfs hasn't
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got any backing store. Because of this, only root (or a trusted user) should
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be allowed write access to a ramfs mount.
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A ramfs derivative called tmpfs was created to add size limits, and the ability
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to write the data to swap space. Normal users can be allowed write access to
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tmpfs mounts. See Documentation/filesystems/tmpfs.txt for more information.
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What is rootfs?
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---------------
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Rootfs is a special instance of ramfs (or tmpfs, if that's enabled), which is
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always present in 2.6 systems. You can't unmount rootfs for approximately the
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same reason you can't kill the init process; rather than having special code
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to check for and handle an empty list, it's smaller and simpler for the kernel
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to just make sure certain lists can't become empty.
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Most systems just mount another filesystem over rootfs and ignore it. The
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amount of space an empty instance of ramfs takes up is tiny.
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What is initramfs?
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------------------
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All 2.6 Linux kernels contain a gzipped "cpio" format archive, which is
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extracted into rootfs when the kernel boots up. After extracting, the kernel
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checks to see if rootfs contains a file "init", and if so it executes it as PID
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1. If found, this init process is responsible for bringing the system the
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rest of the way up, including locating and mounting the real root device (if
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any). If rootfs does not contain an init program after the embedded cpio
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archive is extracted into it, the kernel will fall through to the older code
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to locate and mount a root partition, then exec some variant of /sbin/init
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out of that.
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All this differs from the old initrd in several ways:
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- The old initrd was always a separate file, while the initramfs archive is
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linked into the linux kernel image. (The directory linux-*/usr is devoted
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to generating this archive during the build.)
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- The old initrd file was a gzipped filesystem image (in some file format,
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such as ext2, that needed a driver built into the kernel), while the new
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initramfs archive is a gzipped cpio archive (like tar only simpler,
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see cpio(1) and Documentation/early-userspace/buffer-format.txt). The
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kernel's cpio extraction code is not only extremely small, it's also
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__init data that can be discarded during the boot process.
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- The program run by the old initrd (which was called /initrd, not /init) did
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some setup and then returned to the kernel, while the init program from
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initramfs is not expected to return to the kernel. (If /init needs to hand
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off control it can overmount / with a new root device and exec another init
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program. See the switch_root utility, below.)
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- When switching another root device, initrd would pivot_root and then
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umount the ramdisk. But initramfs is rootfs: you can neither pivot_root
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rootfs, nor unmount it. Instead delete everything out of rootfs to
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free up the space (find -xdev / -exec rm '{}' ';'), overmount rootfs
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with the new root (cd /newmount; mount --move . /; chroot .), attach
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stdin/stdout/stderr to the new /dev/console, and exec the new init.
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Since this is a remarkably persnickity process (and involves deleting
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commands before you can run them), the klibc package introduced a helper
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program (utils/run_init.c) to do all this for you. Most other packages
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(such as busybox) have named this command "switch_root".
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Populating initramfs:
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---------------------
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The 2.6 kernel build process always creates a gzipped cpio format initramfs
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archive and links it into the resulting kernel binary. By default, this
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archive is empty (consuming 134 bytes on x86).
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The config option CONFIG_INITRAMFS_SOURCE (for some reason buried under
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devices->block devices in menuconfig, and living in usr/Kconfig) can be used
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to specify a source for the initramfs archive, which will automatically be
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incorporated into the resulting binary. This option can point to an existing
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gzipped cpio archive, a directory containing files to be archived, or a text
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file specification such as the following example:
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dir /dev 755 0 0
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nod /dev/console 644 0 0 c 5 1
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nod /dev/loop0 644 0 0 b 7 0
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dir /bin 755 1000 1000
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slink /bin/sh busybox 777 0 0
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file /bin/busybox initramfs/busybox 755 0 0
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dir /proc 755 0 0
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dir /sys 755 0 0
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dir /mnt 755 0 0
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file /init initramfs/init.sh 755 0 0
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Run "usr/gen_init_cpio" (after the kernel build) to get a usage message
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documenting the above file format.
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One advantage of the configuration file is that root access is not required to
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set permissions or create device nodes in the new archive. (Note that those
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two example "file" entries expect to find files named "init.sh" and "busybox" in
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a directory called "initramfs", under the linux-2.6.* directory. See
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Documentation/early-userspace/README for more details.)
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The kernel does not depend on external cpio tools. If you specify a
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directory instead of a configuration file, the kernel's build infrastructure
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creates a configuration file from that directory (usr/Makefile calls
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scripts/gen_initramfs_list.sh), and proceeds to package up that directory
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using the config file (by feeding it to usr/gen_init_cpio, which is created
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from usr/gen_init_cpio.c). The kernel's build-time cpio creation code is
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entirely self-contained, and the kernel's boot-time extractor is also
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(obviously) self-contained.
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The one thing you might need external cpio utilities installed for is creating
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or extracting your own preprepared cpio files to feed to the kernel build
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(instead of a config file or directory).
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The following command line can extract a cpio image (either by the above script
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or by the kernel build) back into its component files:
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cpio -i -d -H newc -F initramfs_data.cpio --no-absolute-filenames
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The following shell script can create a prebuilt cpio archive you can
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use in place of the above config file:
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#!/bin/sh
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# Copyright 2006 Rob Landley <rob@landley.net> and TimeSys Corporation.
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# Licensed under GPL version 2
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if [ $# -ne 2 ]
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then
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echo "usage: mkinitramfs directory imagename.cpio.gz"
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exit 1
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fi
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if [ -d "$1" ]
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then
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echo "creating $2 from $1"
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(cd "$1"; find . | cpio -o -H newc | gzip) > "$2"
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else
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echo "First argument must be a directory"
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exit 1
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fi
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Note: The cpio man page contains some bad advice that will break your initramfs
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archive if you follow it. It says "A typical way to generate the list
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of filenames is with the find command; you should give find the -depth option
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to minimize problems with permissions on directories that are unwritable or not
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searchable." Don't do this when creating initramfs.cpio.gz images, it won't
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work. The Linux kernel cpio extractor won't create files in a directory that
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doesn't exist, so the directory entries must go before the files that go in
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those directories. The above script gets them in the right order.
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External initramfs images:
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--------------------------
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If the kernel has initrd support enabled, an external cpio.gz archive can also
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be passed into a 2.6 kernel in place of an initrd. In this case, the kernel
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will autodetect the type (initramfs, not initrd) and extract the external cpio
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archive into rootfs before trying to run /init.
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This has the memory efficiency advantages of initramfs (no ramdisk block
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device) but the separate packaging of initrd (which is nice if you have
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non-GPL code you'd like to run from initramfs, without conflating it with
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the GPL licensed Linux kernel binary).
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It can also be used to supplement the kernel's built-in initamfs image. The
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files in the external archive will overwrite any conflicting files in
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the built-in initramfs archive. Some distributors also prefer to customize
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a single kernel image with task-specific initramfs images, without recompiling.
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Contents of initramfs:
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----------------------
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An initramfs archive is a complete self-contained root filesystem for Linux.
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If you don't already understand what shared libraries, devices, and paths
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you need to get a minimal root filesystem up and running, here are some
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references:
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http://www.tldp.org/HOWTO/Bootdisk-HOWTO/
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http://www.tldp.org/HOWTO/From-PowerUp-To-Bash-Prompt-HOWTO.html
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http://www.linuxfromscratch.org/lfs/view/stable/
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The "klibc" package (http://www.kernel.org/pub/linux/libs/klibc) is
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designed to be a tiny C library to statically link early userspace
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code against, along with some related utilities. It is BSD licensed.
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I use uClibc (http://www.uclibc.org) and busybox (http://www.busybox.net)
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myself. These are LGPL and GPL, respectively. (A self-contained initramfs
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package is planned for the busybox 1.3 release.)
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In theory you could use glibc, but that's not well suited for small embedded
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uses like this. (A "hello world" program statically linked against glibc is
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over 400k. With uClibc it's 7k. Also note that glibc dlopens libnss to do
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name lookups, even when otherwise statically linked.)
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A good first step is to get initramfs to run a statically linked "hello world"
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program as init, and test it under an emulator like qemu (www.qemu.org) or
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User Mode Linux, like so:
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cat > hello.c << EOF
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#include <stdio.h>
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#include <unistd.h>
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int main(int argc, char *argv[])
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{
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printf("Hello world!\n");
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sleep(999999999);
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}
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EOF
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gcc -static hello2.c -o init
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echo init | cpio -o -H newc | gzip > test.cpio.gz
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# Testing external initramfs using the initrd loading mechanism.
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qemu -kernel /boot/vmlinuz -initrd test.cpio.gz /dev/zero
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When debugging a normal root filesystem, it's nice to be able to boot with
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"init=/bin/sh". The initramfs equivalent is "rdinit=/bin/sh", and it's
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just as useful.
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Why cpio rather than tar?
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-------------------------
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This decision was made back in December, 2001. The discussion started here:
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http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1538.html
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And spawned a second thread (specifically on tar vs cpio), starting here:
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http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1587.html
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The quick and dirty summary version (which is no substitute for reading
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the above threads) is:
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1) cpio is a standard. It's decades old (from the AT&T days), and already
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widely used on Linux (inside RPM, Red Hat's device driver disks). Here's
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a Linux Journal article about it from 1996:
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http://www.linuxjournal.com/article/1213
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It's not as popular as tar because the traditional cpio command line tools
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require _truly_hideous_ command line arguments. But that says nothing
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either way about the archive format, and there are alternative tools,
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such as:
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http://freshmeat.net/projects/afio/
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2) The cpio archive format chosen by the kernel is simpler and cleaner (and
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thus easier to create and parse) than any of the (literally dozens of)
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various tar archive formats. The complete initramfs archive format is
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explained in buffer-format.txt, created in usr/gen_init_cpio.c, and
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extracted in init/initramfs.c. All three together come to less than 26k
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total of human-readable text.
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3) The GNU project standardizing on tar is approximately as relevant as
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Windows standardizing on zip. Linux is not part of either, and is free
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to make its own technical decisions.
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4) Since this is a kernel internal format, it could easily have been
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something brand new. The kernel provides its own tools to create and
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extract this format anyway. Using an existing standard was preferable,
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but not essential.
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5) Al Viro made the decision (quote: "tar is ugly as hell and not going to be
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supported on the kernel side"):
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http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1540.html
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explained his reasoning:
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http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1550.html
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http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1638.html
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and, most importantly, designed and implemented the initramfs code.
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Future directions:
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------------------
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Today (2.6.16), initramfs is always compiled in, but not always used. The
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kernel falls back to legacy boot code that is reached only if initramfs does
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not contain an /init program. The fallback is legacy code, there to ensure a
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smooth transition and allowing early boot functionality to gradually move to
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"early userspace" (I.E. initramfs).
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The move to early userspace is necessary because finding and mounting the real
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root device is complex. Root partitions can span multiple devices (raid or
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separate journal). They can be out on the network (requiring dhcp, setting a
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specific mac address, logging into a server, etc). They can live on removable
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media, with dynamically allocated major/minor numbers and persistent naming
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issues requiring a full udev implementation to sort out. They can be
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compressed, encrypted, copy-on-write, loopback mounted, strangely partitioned,
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and so on.
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This kind of complexity (which inevitably includes policy) is rightly handled
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in userspace. Both klibc and busybox/uClibc are working on simple initramfs
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packages to drop into a kernel build.
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The klibc package has now been accepted into Andrew Morton's 2.6.17-mm tree.
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The kernel's current early boot code (partition detection, etc) will probably
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be migrated into a default initramfs, automatically created and used by the
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kernel build.
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