diff --git a/Misc/NEWS b/Misc/NEWS
index 0481ddc..2fa6104 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -29,6 +29,11 @@
 
 - Issue #10464: netrc now correctly handles lines with embedded '#' characters.
 
+Extension Modules
+-----------------
+
+- Issue #678250: Make mmap flush a noop on ACCESS_READ and ACCESS_COPY.
+
 Tests
 -----
 
diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index b53aae1..d6d1601 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -124,7 +124,8 @@
     if (m_obj->fd >= 0)
         (void) close(m_obj->fd);
     if (m_obj->data!=NULL) {
-        msync(m_obj->data, m_obj->size, MS_SYNC);
+        if (m_obj->access != ACCESS_READ && m_obj->access != ACCESS_COPY)
+            msync(m_obj->data, m_obj->size, MS_SYNC);
         munmap(m_obj->data, m_obj->size);
     }
 #endif /* UNIX */
@@ -559,6 +560,10 @@
         PyErr_SetString(PyExc_ValueError, "flush values out of range");
         return NULL;
     }
+
+    if (self->access == ACCESS_READ || self->access == ACCESS_COPY)
+        return PyLong_FromLong(0);
+
 #ifdef MS_WINDOWS
     return PyInt_FromLong((long) FlushViewOfFile(self->data+offset, size));
 #elif defined(UNIX)