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init: allow mounting arbitrary non-blockdevice filesystems as root 

Currently the only non-blockdevice filesystems that can be used as the
initial root filesystem are NFS and CIFS, which use the magic
"root=/dev/nfs" and "root=/dev/cifs" syntax that requires the root
device file system details to come from filesystem specific kernel
command line options.

Add a little bit of new code that allows to just pass arbitrary
string mount options to any non-blockdevice filesystems so that it can
be mounted as the root file system.

For example a virtiofs root file system can be mounted using the
following syntax:

"root=myfs rootfstype=virtiofs rw"

Based on an earlier patch from Vivek Goyal <vgoyal@redhat.com>.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Christoph Hellwig 2021-07-14 16:23:20 -04:00 committed by Al Viro
parent e24d12b744
commit f9259be6a9
1 changed files with 43 additions and 0 deletions
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43
init/do_mounts.c
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@ -534,6 +534,45 @@ static int __init mount_cifs_root(void)
}
#endif
static bool __init fs_is_nodev(char *fstype)
{
struct file_system_type *fs = get_fs_type(fstype);
bool ret = false;
if (fs) {
ret = !(fs->fs_flags & FS_REQUIRES_DEV);
put_filesystem(fs);
}
return ret;
}
static int __init mount_nodev_root(void)
{
char *fs_names, *fstype;
int err = -EINVAL;
fs_names = (void *)__get_free_page(GFP_KERNEL);
if (!fs_names)
return -EINVAL;
split_fs_names(fs_names, root_fs_names);
for (fstype = fs_names; *fstype; fstype += strlen(fstype) + 1) {
if (!fs_is_nodev(fstype))
continue;
err = do_mount_root(root_device_name, fstype, root_mountflags,
root_mount_data);
if (!err)
break;
if (err != -EACCES && err != -EINVAL)
panic("VFS: Unable to mount root \"%s\" (%s), err=%d\n",
root_device_name, fstype, err);
}
free_page((unsigned long)fs_names);
return err;
}
void __init mount_root(void)
{
#ifdef CONFIG_ROOT_NFS
@ -550,6 +589,10 @@ void __init mount_root(void)
return;
}
#endif
if (ROOT_DEV == 0 && root_device_name && root_fs_names) {
if (mount_nodev_root() == 0)
return;
}
#ifdef CONFIG_BLOCK
{
int err = create_dev("/dev/root", ROOT_DEV);