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    Subject[PATCH 2/2] init: allow mounting arbitrary non-blockdevice filesystems as root
    Date
    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>
    ---
    init/do_mounts.c | 43 +++++++++++++++++++++++++++++++++++++++++++
    1 file changed, 43 insertions(+)

    diff --git a/init/do_mounts.c b/init/do_mounts.c
    index ec32de3ad52b..bdeb90b8d669 100644
    --- 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);
    --
    2.30.2
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