fs: use nullfs unconditionally as the real rootfs

Remove the "nullfs_rootfs" boot parameter and simply always use nullfs.
The mutable rootfs will be mounted on top of it. Systems that don't use
pivot_root() to pivot away from the real rootfs will have an additional
mount stick around but that shouldn't be a problem at all. If it is
we'll rever this commit.

This also simplifies the boot process and removes the need for the
traditional switch_root workarounds.

Suggested-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 4 insertions, 20 deletions

diff --git a/Documentation/filesystems/ramfs-rootfs-initramfs.rst b/Documentation/filesystems/ramfs-rootfs-initramfs.rst
index a8899f849e90..165117a721ce 100644
--- a/Documentation/filesystems/ramfs-rootfs-initramfs.rst
+++ b/Documentation/filesystems/ramfs-rootfs-initramfs.rst
@@ -76,13 +76,8 @@ What is rootfs?
 ---------------
 
 Rootfs is a special instance of ramfs (or tmpfs, if that's enabled), which is
-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
+always present in Linux systems.  The kernel uses an immutable empty filesystem
+called nullfs as the true root of the VFS hierarchy, with the mutable rootfs
 (tmpfs/ramfs) mounted on top of it.  This allows pivot_root() and unmounting
 of the initramfs to work normally.
 
@@ -126,25 +121,14 @@ 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.  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
+    umount the ramdisk.  With nullfs as the true root, 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.
+    This is the preferred method for switching root filesystems.
 
 Populating initramfs:
 ---------------------