8259032: MappedMemorySegmentImpl#makeMappedSegment() ignores Unmapper#pagePosition

Co-authored-by: Uwe Schindler <uschindler@openjdk.org>
Reviewed-by: alanb

Author: mcimadamore

src/java.base/share/classes/sun/nio/ch/FileChannelImpl.java

@@ -889,7 +889,7 @@ private Unmapper(long address, long size, long cap,
 
         @Override
         public long address() {
-            return address;
+            return address + pagePosition;
         }
 
         @Override

test/jdk/java/foreign/TestByteBuffer.java

@@ -464,6 +464,28 @@ public void testBadMapNegativeOffset() throws IOException {
         MemorySegment.mapFile(f.toPath(), -1, 1, FileChannel.MapMode.READ_WRITE);
     }
 
+    @Test
+    public void testMapOffset() throws IOException {
+        File f = new File("testMapOffset.out");
+        f.createNewFile();
+        f.deleteOnExit();
+
+        int SIZE = Byte.MAX_VALUE;
+
+        try (MemorySegment segment = MemorySegment.mapFile(f.toPath(), 0, SIZE, FileChannel.MapMode.READ_WRITE)) {
+            for (byte offset = 0; offset < SIZE; offset++) {
+                MemoryAccess.setByteAtOffset(segment, offset, offset);
+            }
+            MappedMemorySegments.force(segment);
+        }
+
+        for (int offset = 0 ; offset < SIZE ; offset++) {
+            try (MemorySegment segment = MemorySegment.mapFile(f.toPath(), offset, SIZE - offset, FileChannel.MapMode.READ_ONLY)) {
+                assertEquals(MemoryAccess.getByte(segment), offset);
+            }
+        }
+    }
+
     @Test
     public void testMapZeroSize() throws IOException {
         File f = new File("testPos1.out");