diff -r d35dbf596638 -r a8aa2edb7e67 Lib/test/test_mmap.py
--- a/Lib/test/test_mmap.py	Tue Feb 08 00:10:33 2011 +0100
+++ b/Lib/test/test_mmap.py	Wed Feb 09 16:36:59 2011 +0200
@@ -1,4 +1,4 @@
-from test.support import TESTFN, run_unittest, import_module
+from test.support import TESTFN, run_unittest, import_module, unlink
 import unittest
 import os
 import re
@@ -646,9 +646,57 @@
                               "wrong exception raised in context manager")
         self.assertTrue(m.closed, "context manager failed")
 
+class LargeMmapTests(unittest.TestCase):
+
+    def setUp(self):
+        unlink(TESTFN)
+
+    def tearDown(self):
+        unlink(TESTFN)
+
+    def _working_largefile(self):
+        # Only run if the current filesystem supports large files.
+        f = open(TESTFN, 'wb', buffering=0)
+        try:
+            f.seek(0x80000001)
+            f.write(b'x')
+            f.flush()
+        except (IOError, OverflowError):
+            raise unittest.SkipTest("filesystem does not have largefile support")
+        finally:
+            os.unlink(TESTFN)
+            f.close()
+
+    def test_large_offset(self):
+        if sys.platform[:3] == 'win' or sys.platform == 'darwin':
+            requires('largefile',
+                'test requires %s bytes and a long time to run' % str(0x180000000))
+        self._working_largefile()
+        with open (TESTFN, 'wb') as f:
+            f.seek(0x17FFFFFFF)
+            f.write(b" ")
+
+        with open (TESTFN, 'rb') as f:
+            with mmap.mmap(f.fileno(), 0, offset=0x140000000, access=mmap.ACCESS_READ) as m:
+                self.assertEqual(m[0x3FFFFFFF], 32)
+
+    def test_large_filesize(self):
+        if sys.platform[:3] == 'win' or sys.platform == 'darwin':
+            requires('largefile',
+                'test requires %s bytes and a long time to run' % str(0x180000000))
+        self._working_largefile()
+        with open (TESTFN, 'wb') as f:
+            f.seek(0x17FFFFFFF)
+            f.write(b" ")
+
+        with open (TESTFN, 'rb') as f:
+            with mmap.mmap(f.fileno(), 0x10000, access=mmap.ACCESS_READ) as m:
+                self.assertEqual(m.size(), 0x180000000)
+
 
 def test_main():
     run_unittest(MmapTests)
+    run_unittest(LargeMmapTests)
 
 if __name__ == '__main__':
     test_main()
diff -r d35dbf596638 -r a8aa2edb7e67 Modules/mmapmodule.c
--- a/Modules/mmapmodule.c	Tue Feb 08 00:10:33 2011 +0100
+++ b/Modules/mmapmodule.c	Wed Feb 09 16:36:59 2011 +0200
@@ -90,7 +90,7 @@
     char *      data;
     size_t      size;
     size_t      pos;    /* relative to offset */
-    size_t      offset;
+    off_t       offset;
     int     exports;
 
 #ifdef MS_WINDOWS
@@ -433,7 +433,11 @@
             PyErr_SetFromErrno(mmap_module_error);
             return NULL;
         }
-        return PyLong_FromSsize_t(buf.st_size);
+#ifdef HAVE_LARGEFILE_SUPPORT
+        return PyLong_FromLongLong(buf.st_size);
+#else
+        return PyLong_FromLong(buf.st_size);
+#endif
     }
 #endif /* UNIX */
 }
@@ -1051,6 +1055,12 @@
 }
 
 #ifdef UNIX
+#ifdef HAVE_LARGEFILE_SUPPORT
+#define _Py_PARSE_OFF_T "L"
+#else
+#define _Py_PARSE_OFF_T "l"
+#endif
+
 static PyObject *
 new_mmap_object(PyTypeObject *type, PyObject *args, PyObject *kwdict)
 {
@@ -1058,8 +1068,9 @@
     struct stat st;
 #endif
     mmap_object *m_obj;
-    PyObject *map_size_obj = NULL, *offset_obj = NULL;
-    Py_ssize_t map_size, offset;
+    PyObject *map_size_obj = NULL;
+    Py_ssize_t map_size;
+    off_t offset = 0;
     int fd, flags = MAP_SHARED, prot = PROT_WRITE | PROT_READ;
     int devzero = -1;
     int access = (int)ACCESS_DEFAULT;
@@ -1067,16 +1078,18 @@
                                      "flags", "prot",
                                      "access", "offset", NULL};
 
-    if (!PyArg_ParseTupleAndKeywords(args, kwdict, "iO|iiiO", keywords,
+    if (!PyArg_ParseTupleAndKeywords(args, kwdict, "iO|iii" _Py_PARSE_OFF_T, keywords,
                                      &fd, &map_size_obj, &flags, &prot,
-                                     &access, &offset_obj))
+                                     &access, &offset))
         return NULL;
     map_size = _GetMapSize(map_size_obj, "size");
     if (map_size < 0)
         return NULL;
-    offset = _GetMapSize(offset_obj, "offset");
-    if (offset < 0)
+    if (offset < 0) {
+        PyErr_SetString(PyExc_OverflowError,
+            "memory mapped offset must be positive");
         return NULL;
+    }
 
     if ((access != (int)ACCESS_DEFAULT) &&
         ((flags != MAP_SHARED) || (prot != (PROT_WRITE | PROT_READ))))
@@ -1121,8 +1134,14 @@
                                 "mmap offset is greater than file size");
                 return NULL;
             }
-            map_size = st.st_size - offset;
-        } else if ((size_t)offset + (size_t)map_size > st.st_size) {
+            off_t calc_size = st.st_size - offset;
+            map_size = calc_size;
+            if (map_size != calc_size) {
+                PyErr_SetString(PyExc_ValueError,
+                                 "mmap length is too large");
+                 return NULL;
+             }
+        } else if (offset + (size_t)map_size > st.st_size) {
             PyErr_SetString(PyExc_ValueError,
                             "mmap length is greater than file size");
             return NULL;