diff --git a/samples/memfs/fs.go b/samples/memfs/fs.go
index e4dc23e..6908002 100644
--- a/samples/memfs/fs.go
+++ b/samples/memfs/fs.go
@@ -137,10 +137,6 @@ func (fs *memFS) LookUpInode(
 	inode := fs.getInodeForReadingOrDie(req.Parent)
 	defer inode.mu.RUnlock()
 
-	if !inode.dir {
-		panic("Found non-dir.")
-	}
-
 	// Does the directory have an entry with the given name?
 	childID, ok := inode.LookUpChild(req.Name)
 	if !ok {
@@ -223,15 +219,11 @@ func (fs *memFS) ReadDir(
 		panic("Found non-dir.")
 	}
 
-	// Return the entries requested.
-	for i := int(req.Offset); i < len(inode.entries); i++ {
-		resp.Data = fuseutil.AppendDirent(resp.Data, inode.entries[i])
-
-		// Trim and stop early if we've exceeded the requested size.
-		if len(resp.Data) > req.Size {
-			resp.Data = resp.Data[:req.Size]
-			break
-		}
+	// Serve the request.
+	resp.Data, err = inode.ReadDir(int(req.Offset), req.Size)
+	if err != nil {
+		err = fmt.Errorf("inode.ReadDir: %v", err)
+		return
 	}
 
 	return
diff --git a/samples/memfs/inode.go b/samples/memfs/inode.go
index 5ae3c22..463e56a 100644
--- a/samples/memfs/inode.go
+++ b/samples/memfs/inode.go
@@ -55,7 +55,14 @@ type inode struct {
 	contents []byte // GUARDED_BY(mu)
 }
 
-func newInode(dir bool) (inode *inode)
+func newInode(dir bool) (in *inode) {
+	in = &inode{
+		dir: dir,
+	}
+
+	in.mu = syncutil.NewInvariantMutex(in.checkInvariants)
+	return
+}
 
 func (inode *inode) checkInvariants()
 
@@ -63,4 +70,32 @@ func (inode *inode) checkInvariants()
 //
 // REQUIRES: inode.dir
 // SHARED_LOCKS_REQUIRED(inode.mu)
-func (inode *inode) LookUpChild(name string) (id fuse.InodeID, ok bool)
+func (inode *inode) LookUpChild(name string) (id fuse.InodeID, ok bool) {
+	if !inode.dir {
+		panic("LookUpChild called on non-directory.")
+	}
+
+	panic("TODO")
+}
+
+// Serve a ReadDir request.
+//
+// REQUIRED: inode.dir
+// SHARED_LOCKS_REQUIRED(inode.mu)
+func (inode *inode) ReadDir(offset int, size int) (data []byte, err error) {
+	if !inode.dir {
+		panic("ReadDir called on non-directory.")
+	}
+
+	for i := offset; i < len(inode.entries); i++ {
+		data = fuseutil.AppendDirent(data, inode.entries[i])
+
+		// Trim and stop early if we've exceeded the requested size.
+		if len(data) > size {
+			data = data[:size]
+			break
+		}
+	}
+
+	return
+}