NFS: Fix the ustat() regression

Since 2.6.18, the superblock sb->s_root has been a dummy dentry with a
dummy inode. This breaks ustat(), which actually uses sb->s_root in a
vfstat() call.

Fix this by making the s_root a dummy alias to the directory inode that was
used when creating the superblock.
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/getroot.c

@@ -42,6 +42,25 @@
 
 #define NFSDBG_FACILITY		NFSDBG_CLIENT
 
+/*
+ * Set the superblock root dentry.
+ * Note that this function frees the inode in case of error.
+ */
+static int nfs_superblock_set_dummy_root(struct super_block *sb, struct inode *inode)
+{
+	/* The mntroot acts as the dummy root dentry for this superblock */
+	if (sb->s_root == NULL) {
+		sb->s_root = d_alloc_root(inode);
+		if (sb->s_root == NULL) {
+			iput(inode);
+			return -ENOMEM;
+		}
+		/* Circumvent igrab(): we know the inode is not being freed */
+		atomic_inc(&inode->i_count);
+	}
+	return 0;
+}
+
 /*
  * get an NFS2/NFS3 root dentry from the root filehandle
  */
@@ -54,33 +73,6 @@ struct dentry *nfs_get_root(struct super_block *sb, struct nfs_fh *mntfh)
 	struct inode *inode;
 	int error;
 
-	/* create a dummy root dentry with dummy inode for this superblock */
-	if (!sb->s_root) {
-		struct nfs_fh dummyfh;
-		struct dentry *root;
-		struct inode *iroot;
-
-		memset(&dummyfh, 0, sizeof(dummyfh));
-		memset(&fattr, 0, sizeof(fattr));
-		nfs_fattr_init(&fattr);
-		fattr.valid = NFS_ATTR_FATTR;
-		fattr.type = NFDIR;
-		fattr.mode = S_IFDIR | S_IRUSR | S_IWUSR;
-		fattr.nlink = 2;
-
-		iroot = nfs_fhget(sb, &dummyfh, &fattr);
-		if (IS_ERR(iroot))
-			return ERR_PTR(PTR_ERR(iroot));
-
-		root = d_alloc_root(iroot);
-		if (!root) {
-			iput(iroot);
-			return ERR_PTR(-ENOMEM);
-		}
-
-		sb->s_root = root;
-	}
-
 	/* get the actual root for this mount */
 	fsinfo.fattr = &fattr;
 
@@ -96,6 +88,10 @@ struct dentry *nfs_get_root(struct super_block *sb, struct nfs_fh *mntfh)
 		return ERR_PTR(PTR_ERR(inode));
 	}
 
+	error = nfs_superblock_set_dummy_root(sb, inode);
+	if (error != 0)
+		return ERR_PTR(error);
+
 	/* root dentries normally start off anonymous and get spliced in later
 	 * if the dentry tree reaches them; however if the dentry already
 	 * exists, we'll pick it up at this point and use it as the root
@@ -241,33 +237,6 @@ struct dentry *nfs4_get_root(struct super_block *sb, struct nfs_fh *mntfh)
 
 	dprintk("--> nfs4_get_root()\n");
 
-	/* create a dummy root dentry with dummy inode for this superblock */
-	if (!sb->s_root) {
-		struct nfs_fh dummyfh;
-		struct dentry *root;
-		struct inode *iroot;
-
-		memset(&dummyfh, 0, sizeof(dummyfh));
-		memset(&fattr, 0, sizeof(fattr));
-		nfs_fattr_init(&fattr);
-		fattr.valid = NFS_ATTR_FATTR;
-		fattr.type = NFDIR;
-		fattr.mode = S_IFDIR | S_IRUSR | S_IWUSR;
-		fattr.nlink = 2;
-
-		iroot = nfs_fhget(sb, &dummyfh, &fattr);
-		if (IS_ERR(iroot))
-			return ERR_PTR(PTR_ERR(iroot));
-
-		root = d_alloc_root(iroot);
-		if (!root) {
-			iput(iroot);
-			return ERR_PTR(-ENOMEM);
-		}
-
-		sb->s_root = root;
-	}
-
 	/* get the info about the server and filesystem */
 	error = nfs4_server_capabilities(server, mntfh);
 	if (error < 0) {
@@ -289,6 +258,10 @@ struct dentry *nfs4_get_root(struct super_block *sb, struct nfs_fh *mntfh)
 		return ERR_PTR(PTR_ERR(inode));
 	}
 
+	error = nfs_superblock_set_dummy_root(sb, inode);
+	if (error != 0)
+		return ERR_PTR(error);
+
 	/* root dentries normally start off anonymous and get spliced in later
 	 * if the dentry tree reaches them; however if the dentry already
 	 * exists, we'll pick it up at this point and use it as the root