diff --git a/prometheus_client/mmap_dict.py b/prometheus_client/mmap_dict.py
index edd895cd..fb4a19eb 100644
--- a/prometheus_client/mmap_dict.py
+++ b/prometheus_client/mmap_dict.py
@@ -101,7 +101,8 @@ def _init_value(self, key):
         while self._used + len(value) > self._capacity:
             self._capacity *= 2
             self._f.truncate(self._capacity)
-            self._m = mmap.mmap(self._f.fileno(), self._capacity)
+            self._m.close()
+            self._m = mmap.mmap(self._f.fileno(), self._capacity, access=mmap.ACCESS_WRITE)
         self._m[self._used:self._used + len(value)] = value
 
         # Update how much space we've used.
@@ -131,12 +132,21 @@ def write_value(self, key, value, timestamp):
         _pack_two_doubles(self._m, pos, value, timestamp)
 
     def close(self):
-        if self._f:
-            self._m.close()
+        m = getattr(self, '_m', None)
+        if m is not None:
+            m.close()
             self._m = None
-            self._f.close()
+        f = getattr(self, '_f', None)
+        if f is not None:
+            f.close()
             self._f = None
 
+    def __del__(self):
+        try:
+            self.close()
+        except Exception:
+            return
+
 
 def mmap_key(metric_name: str, name: str, labelnames: List[str], labelvalues: List[str], help_text: str) -> str:
     """Format a key for use in the mmap file."""
diff --git a/tests/test_mmap_dict_close.py b/tests/test_mmap_dict_close.py
new file mode 100644
index 00000000..db5adcbe
--- /dev/null
+++ b/tests/test_mmap_dict_close.py
@@ -0,0 +1,47 @@
+import gc
+import os
+import tempfile
+import unittest
+import warnings
+
+import prometheus_client.mmap_dict as mmap_dict
+from prometheus_client.mmap_dict import MmapedDict
+
+
+class TestMmapedDictClose(unittest.TestCase):
+    def test_discarded_dict_does_not_warn_about_an_open_file(self):
+        with tempfile.TemporaryDirectory() as tmp:
+            path = os.path.join(tmp, "metrics.db")
+            with warnings.catch_warnings(record=True) as caught:
+                warnings.simplefilter("always", ResourceWarning)
+                store = MmapedDict(path)
+                store.write_value("one", 1.0, 2.0)
+                del store
+                gc.collect()
+        self.assertEqual(
+            [w for w in caught if issubclass(w.category, ResourceWarning)],
+            [],
+        )
+
+    def test_growth_keeps_written_values(self):
+        mmap_dict._INITIAL_MMAP_SIZE = 128
+        try:
+            with tempfile.TemporaryDirectory() as tmp:
+                path = os.path.join(tmp, "metrics.db")
+                store = MmapedDict(path)
+                for i in range(40):
+                    store.write_value("k" * 20 + str(i), float(i), 1.0)
+                store.close()
+                values = {
+                    key: value
+                    for key, value, _, _ in MmapedDict.read_all_values_from_file(path)
+                }
+        finally:
+            mmap_dict._INITIAL_MMAP_SIZE = 1 << 16
+        self.assertEqual(values["k" * 20 + "0"], 0.0)
+        self.assertEqual(values["k" * 20 + "39"], 39.0)
+        self.assertEqual(len(values), 40)
+
+
+if __name__ == "__main__":
+    unittest.main()