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>

3 files changed, 40 insertions, 53 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;
 }
 
diff --git a/fs/namespace.c b/fs/namespace.c
index aea4b7689840..50ee30345b4f 100644
--- a/fs/namespace.c
+++ b/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)
 {
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;
 }