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>

1 files changed, 6 insertions, 29 deletions

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index cfbedd7755b0..17ee7fc5e64e 100644
--- a/fs/btrfs/super.c
+++ b/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;
 }