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docs: mention nullfs

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Add a section about nullfs and how it enables pivot_root() to work.

Link: https://patch.msgid.link/20260112-work-immutable-rootfs-v2-4-88dd1c34a204@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>
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Christian Brauner 2026-01-12 16:47:11 +01:00
parent 576ee5dfd4
commit 649cb20b7a
1 changed files with 23 additions and 9 deletions
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32
Documentation/filesystems/ramfs-rootfs-initramfs.rst
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@ -76,10 +76,15 @@ 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.
always present in 2.6 systems. Traditionally, 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.
However, if the kernel is booted with "nullfs_rootfs", an immutable empty
filesystem called nullfs is used as the true root, with the mutable rootfs
(tmpfs/ramfs) mounted on top of it. This allows pivot_root() and unmounting
of the initramfs to work normally.
Most systems just mount another filesystem over rootfs and ignore it. The
amount of space an empty instance of ramfs takes up is tiny.
@ -121,17 +126,26 @@ All this differs from the old initrd in several ways:
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.
umount the ramdisk. Traditionally, 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".
However, if the kernel is booted with "nullfs_rootfs", pivot_root() works
normally from the initramfs. Userspace can simply do::
chdir(new_root);
pivot_root(".", ".");
umount2(".", MNT_DETACH);
This is the preferred method when nullfs_rootfs is enabled.
Populating initramfs:
---------------------