new helper: mount_subtree()

takes vfsmount and relative path, does lookup within that vfsmount
(possibly triggering automounts) and returns the result as root
of subtree suitable for return by ->mount() (i.e. a reference to
dentry and an active reference to its superblock grabbed, superblock
locked exclusive).

btrfs and nfs switched to it instead of open-coding the sucker.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/btrfs/super.c

@@ -825,13 +825,9 @@ static char *setup_root_args(char *args)
 static struct dentry *mount_subvol(const char *subvol_name, int flags,
 				   const char *device_name, char *data)
 {
-	struct super_block *s;
 	struct dentry *root;
 	struct vfsmount *mnt;
-	struct mnt_namespace *ns_private;
 	char *newargs;
-	struct path path;
-	int error;
 
 	newargs = setup_root_args(data);
 	if (!newargs)
@@ -842,36 +838,17 @@ static struct dentry *mount_subvol(const char *subvol_name, int flags,
 	if (IS_ERR(mnt))
 		return ERR_CAST(mnt);
 
-	ns_private = create_mnt_ns(mnt);
-	if (IS_ERR(ns_private))
-		return ERR_CAST(ns_private);
-
-	/*
-	 * This will trigger the automount of the subvol so we can just
-	 * drop the mnt we have here and return the dentry that we
-	 * found.
-	 */
-	error = vfs_path_lookup(mnt->mnt_root, mnt, subvol_name,
-				LOOKUP_FOLLOW, &path);
-	put_mnt_ns(ns_private);
-	if (error)
-		return ERR_PTR(error);
+	root = mount_subtree(mnt, subvol_name);
 
-	if (!is_subvolume_inode(path.dentry->d_inode)) {
-		path_put(&path);
-		error = -EINVAL;
+	if (!IS_ERR(root) && !is_subvolume_inode(root->d_inode)) {
+		struct super_block *s = root->d_sb;
+		dput(root);
+		root = ERR_PTR(-EINVAL);
+		deactivate_locked_super(s);
 		printk(KERN_ERR "btrfs: '%s' is not a valid subvolume\n",
 				subvol_name);
-		return ERR_PTR(-EINVAL);
 	}
 
-	/* Get a ref to the sb and the dentry we found and return it */
-	s = path.mnt->mnt_sb;
-	atomic_inc(&s->s_active);
-	root = dget(path.dentry);
-	path_put(&path);
-	down_write(&s->s_umount);
-
 	return root;
 }
 

fs/namespace.c

@@ -2490,6 +2490,34 @@ struct mnt_namespace *create_mnt_ns(struct vfsmount *mnt)
 }
 EXPORT_SYMBOL(create_mnt_ns);
 
+struct dentry *mount_subtree(struct vfsmount *mnt, const char *name)
+{
+	struct mnt_namespace *ns;
+	struct path path;
+	int err;
+
+	ns = create_mnt_ns(mnt);
+	if (IS_ERR(ns))
+		return ERR_CAST(ns);
+
+	err = vfs_path_lookup(mnt->mnt_root, mnt,
+			name, LOOKUP_FOLLOW|LOOKUP_AUTOMOUNT, &path);
+
+	put_mnt_ns(ns);
+
+	if (err)
+		return ERR_PTR(err);
+
+	/* trade a vfsmount reference for active sb one */
+	atomic_inc(&path.mnt->mnt_sb->s_active);
+	mntput(path.mnt);
+	/* lock the sucker */
+	down_write(&path.mnt->mnt_sb->s_umount);
+	/* ... and return the root of (sub)tree on it */
+	return path.dentry;
+}
+EXPORT_SYMBOL(mount_subtree);
+
 SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
 		char __user *, type, unsigned long, flags, void __user *, data)
 {

fs/nfs/super.c

@@ -2787,35 +2787,17 @@ static void nfs_referral_loop_unprotect(void)
 static struct dentry *nfs_follow_remote_path(struct vfsmount *root_mnt,
 		const char *export_path)
 {
-	struct mnt_namespace *ns_private;
-	struct super_block *s;
 	struct dentry *dentry;
-	struct path path;
-	int ret;
-
-	ns_private = create_mnt_ns(root_mnt);
-	if (IS_ERR(ns_private))
-		return ERR_CAST(ns_private);
+	int ret = nfs_referral_loop_protect();
 
-	ret = nfs_referral_loop_protect();
-	if (ret == 0) {
-		ret = vfs_path_lookup(root_mnt->mnt_root, root_mnt,
-				export_path, LOOKUP_FOLLOW|LOOKUP_AUTOMOUNT,
-				&path);
-		nfs_referral_loop_unprotect();
-	}
-
-	put_mnt_ns(ns_private);
-
-	if (ret != 0)
+	if (ret) {
+		mntput(root_mnt);
 		return ERR_PTR(ret);
+	}
 
-	s = path.mnt->mnt_sb;
-	atomic_inc(&s->s_active);
-	dentry = dget(path.dentry);
+	dentry = mount_subtree(root_mnt, export_path);
+	nfs_referral_loop_unprotect();
 
-	path_put(&path);
-	down_write(&s->s_umount);
 	return dentry;
 }
 

include/linux/fs.h

@@ -1886,6 +1886,7 @@ extern struct dentry *mount_single(struct file_system_type *fs_type,
 extern struct dentry *mount_nodev(struct file_system_type *fs_type,
 	int flags, void *data,
 	int (*fill_super)(struct super_block *, void *, int));
+extern struct dentry *mount_subtree(struct vfsmount *mnt, const char *path);
 void generic_shutdown_super(struct super_block *sb);
 void kill_block_super(struct super_block *sb);
 void kill_anon_super(struct super_block *sb);