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>

diff --git a/init/do_mounts.c b/init/do_mounts.c
index ec32de3..bdeb90b 100644 (file)
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -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);