diff --git a/mercurial/cext/dirs.c b/mercurial/cext/dirs.c
--- a/mercurial/cext/dirs.c
+++ b/mercurial/cext/dirs.c
@@ -48,12 +48,30 @@
 	return pos;
 }
 
+/* Mercurial will fail to run on directory hierarchies deeper than
+ * this constant, so we should try and keep this constant as big as
+ * possible.
+ */
+#define MAX_DIRS_DEPTH 2048
+
 static int _addpath(PyObject *dirs, PyObject *path)
 {
 	const char *cpath = PyBytes_AS_STRING(path);
 	Py_ssize_t pos = PyBytes_GET_SIZE(path);
 	PyObject *key = NULL;
 	int ret = -1;
+	size_t num_slashes = 0, i;
+
+	for (i = 0; i < pos; ++i) {
+		if (cpath[i] == '/') {
+			++num_slashes;
+		}
+	}
+	if (num_slashes > MAX_DIRS_DEPTH) {
+		PyErr_SetString(PyExc_ValueError,
+		                "Directory hierarchy too deep.");
+		goto bail;
+	}
 
 	/* This loop is super critical for performance. That's why we inline
 	 * access to Python structs instead of going through a supported API.