devtmpfs: don't mix {ramfs,shmem}_fill_super() with mount_single()

Create an internal-only type matching the current devtmpfs, never
register it and have one kernel-internal mount done.  That thing
gets mounted only once, so it is free to use mount_nodev().

The "public" devtmpfs (the one we do register, and only after
the internal mount of the real thing is done) simply gets and
returns an extra reference to the internal superblock.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

drivers/base/devtmpfs.c

@@ -56,22 +56,38 @@ static int __init mount_param(char *str)
 }
 __setup("devtmpfs.mount=", mount_param);
 
+static struct vfsmount *mnt;
+
+static struct dentry *public_dev_mount(struct file_system_type *fs_type, int flags,
+		      const char *dev_name, void *data)
+{
+	struct super_block *s = mnt->mnt_sb;
+	atomic_inc(&s->s_active);
+	down_write(&s->s_umount);
+	return dget(s->s_root);
+}
+
 static struct dentry *dev_mount(struct file_system_type *fs_type, int flags,
 		      const char *dev_name, void *data)
 {
 #ifdef CONFIG_TMPFS
-	return mount_single(fs_type, flags, data, shmem_fill_super);
+	return mount_nodev(fs_type, flags, data, shmem_fill_super);
 #else
-	return mount_single(fs_type, flags, data, ramfs_fill_super);
+	return ramfs_mount(fs_type, flags, dev_name, data);
 #endif
 }
 
-static struct file_system_type dev_fs_type = {
+static struct file_system_type internal_fs_type = {
 	.name = "devtmpfs",
 	.mount = dev_mount,
 	.kill_sb = kill_litter_super,
 };
 
+static struct file_system_type dev_fs_type = {
+	.name = "devtmpfs",
+	.mount = public_dev_mount,
+};
+
 #ifdef CONFIG_BLOCK
 static inline int is_blockdev(struct device *dev)
 {
@@ -378,12 +394,11 @@ static int handle(const char *name, umode_t mode, kuid_t uid, kgid_t gid,
 
 static int devtmpfsd(void *p)
 {
-	char options[] = "mode=0755";
 	int *err = p;
 	*err = ksys_unshare(CLONE_NEWNS);
 	if (*err)
 		goto out;
-	*err = ksys_mount("devtmpfs", "/", "devtmpfs", MS_SILENT, options);
+	*err = ksys_mount("devtmpfs", "/", "devtmpfs", MS_SILENT, NULL);
 	if (*err)
 		goto out;
 	ksys_chdir("/.."); /* will traverse into overmounted root */
@@ -420,7 +435,16 @@ static int devtmpfsd(void *p)
  */
 int __init devtmpfs_init(void)
 {
-	int err = register_filesystem(&dev_fs_type);
+	char opts[] = "mode=0755";
+	int err;
+
+	mnt = vfs_kern_mount(&internal_fs_type, 0, "devtmpfs", opts);
+	if (IS_ERR(mnt)) {
+		printk(KERN_ERR "devtmpfs: unable to create devtmpfs %ld\n",
+				PTR_ERR(mnt));
+		return PTR_ERR(mnt);
+	}
+	err = register_filesystem(&dev_fs_type);
 	if (err) {
 		printk(KERN_ERR "devtmpfs: unable to register devtmpfs "
 		       "type %i\n", err);