Add a section about nullfs and how it enables pivot_root() to work. Signed-off-by: Christian Brauner --- .../filesystems/ramfs-rootfs-initramfs.rst | 32 ++++++++++++++++------ 1 file changed, 23 insertions(+), 9 deletions(-) diff --git a/Documentation/filesystems/ramfs-rootfs-initramfs.rst b/Documentation/filesystems/ramfs-rootfs-initramfs.rst index a9d271e171c3..a8899f849e90 100644 --- a/Documentation/filesystems/ramfs-rootfs-initramfs.rst +++ b/Documentation/filesystems/ramfs-rootfs-initramfs.rst @@ -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: --------------------- -- 2.47.3