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, 24 deletions

diff --git a/fs/nfs/super.c b/fs/nfs/super.c
index 46d69f38fd55..134777406ee3 100644
--- a/fs/nfs/super.c
+++ b/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;
+        int ret = nfs_referral_loop_protect();
 
-        ns_private = create_mnt_ns(root_mnt);
-        if (IS_ERR(ns_private))
-                return ERR_CAST(ns_private);
-
-        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;
 }