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fs: use nullfs unconditionally as the real rootfs

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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>
This commit is contained in:
Christian Brauner
2026-01-14 11:22:54 +01:00
parent 7416634fd6
commit 313c47f4fe
4 changed files with 32 additions and 77 deletions
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64
fs/namespace.c
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@@ -75,17 +75,6 @@ static int __init initramfs_options_setup(char *str)
__setup("initramfs_options=", initramfs_options_setup);
bool nullfs_rootfs = false;
static int __init nullfs_rootfs_setup(char *str)
{
if (*str)
return 0;
nullfs_rootfs = true;
return 1;
}
__setup("nullfs_rootfs", nullfs_rootfs_setup);
static u64 event;
static DEFINE_XARRAY_FLAGS(mnt_id_xa, XA_FLAGS_ALLOC);
static DEFINE_IDA(mnt_group_ida);
@@ -4593,10 +4582,9 @@ int path_pivot_root(struct path *new, struct path *old)
* pointed to by put_old must yield the same directory as new_root. No other
* file system may be mounted on put_old. After all, new_root is a mountpoint.
*
* Also, the current root cannot be on the 'rootfs' (initial ramfs) filesystem
* unless the kernel was booted with "nullfs_rootfs". See
* Documentation/filesystems/ramfs-rootfs-initramfs.rst for alternatives
* in this situation.
* The immutable nullfs filesystem is mounted as the true root of the VFS
* hierarchy. The mutable rootfs (tmpfs/ramfs) is layered on top of this,
* allowing pivot_root() to work normally from initramfs.
*
* Notes:
* - we don't move root/cwd if they are not at the root (reason: if something
@@ -5993,49 +5981,39 @@ static void __init init_mount_tree(void)
struct path root;
/*
* When nullfs is used, we create two mounts:
* We create two mounts:
*
* (1) nullfs with mount id 1
* (2) mutable rootfs with mount id 2
*
* with (2) mounted on top of (1).
*/
if (nullfs_rootfs) {
nullfs_mnt = vfs_kern_mount(&nullfs_fs_type, 0, "nullfs", NULL);
if (IS_ERR(nullfs_mnt))
panic("VFS: Failed to create nullfs");
}
nullfs_mnt = vfs_kern_mount(&nullfs_fs_type, 0, "nullfs", NULL);
if (IS_ERR(nullfs_mnt))
panic("VFS: Failed to create nullfs");
mnt = vfs_kern_mount(&rootfs_fs_type, 0, "rootfs", initramfs_options);
if (IS_ERR(mnt))
panic("Can't create rootfs");
if (nullfs_rootfs) {
VFS_WARN_ON_ONCE(real_mount(nullfs_mnt)->mnt_id != 1);
VFS_WARN_ON_ONCE(real_mount(mnt)->mnt_id != 2);
VFS_WARN_ON_ONCE(real_mount(nullfs_mnt)->mnt_id != 1);
VFS_WARN_ON_ONCE(real_mount(mnt)->mnt_id != 2);
/* The namespace root is the nullfs mnt. */
mnt_root = real_mount(nullfs_mnt);
init_mnt_ns.root = mnt_root;
/* The namespace root is the nullfs mnt. */
mnt_root = real_mount(nullfs_mnt);
init_mnt_ns.root = mnt_root;
/* Mount mutable rootfs on top of nullfs. */
root.mnt = nullfs_mnt;
root.dentry = nullfs_mnt->mnt_root;
/* Mount mutable rootfs on top of nullfs. */
root.mnt = nullfs_mnt;
root.dentry = nullfs_mnt->mnt_root;
LOCK_MOUNT_EXACT(mp, &root);
if (unlikely(IS_ERR(mp.parent)))
panic("VFS: Failed to mount rootfs on nullfs");
scoped_guard(mount_writer)
attach_mnt(real_mount(mnt), mp.parent, mp.mp);
LOCK_MOUNT_EXACT(mp, &root);
if (unlikely(IS_ERR(mp.parent)))
panic("VFS: Failed to mount rootfs on nullfs");
scoped_guard(mount_writer)
attach_mnt(real_mount(mnt), mp.parent, mp.mp);
pr_info("VFS: Finished mounting rootfs on nullfs\n");
} else {
VFS_WARN_ON_ONCE(real_mount(mnt)->mnt_id != 1);
/* The namespace root is the mutable rootfs. */
mnt_root = real_mount(mnt);
init_mnt_ns.root = mnt_root;
}
pr_info("VFS: Finished mounting rootfs on nullfs\n");
/*
* We've dropped all locks here but that's fine. Not just are we