--- old/src/jdk.incubator.foreign/share/classes/jdk/internal/foreign/MappedMemorySegmentImpl.java	2020-04-30 16:04:02.000000000 +0100
+++ new/src/jdk.incubator.foreign/share/classes/jdk/internal/foreign/MappedMemorySegmentImpl.java	2020-04-30 16:04:02.000000000 +0100
@@ -104,8 +104,12 @@
         try (FileChannelImpl channelImpl = (FileChannelImpl)FileChannel.open(path, openOptions(mapMode))) {
             UnmapperProxy unmapperProxy = channelImpl.mapInternal(mapMode, 0L, bytesSize);
             MemoryScope scope = new MemoryScope(null, unmapperProxy::unmap);
+            int modes = defaultAccessModes(bytesSize);
+            if (mapMode == FileChannel.MapMode.READ_ONLY) {
+                modes &= ~WRITE;
+            }
             return new MappedMemorySegmentImpl(unmapperProxy.address(), unmapperProxy, bytesSize,
-                    defaultAccessModes(bytesSize), Thread.currentThread(), scope);
+                    modes, Thread.currentThread(), scope);
         }
     }