diff --git a/hgext3rd/extmmap.pyx b/hgext3rd/extmmap.pyx
new file mode 100644
--- /dev/null
+++ b/hgext3rd/extmmap.pyx
@@ -0,0 +1,70 @@
+# extmmap.pyx - Simple mmap implementation that does not keep a fd open
+#
+# Copyright 2017 Facebook, Inc.
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+from cpython.bytes cimport PyBytes_FromStringAndSize
+from libc.errno cimport errno
+from posix cimport mman, stat
+
+import mmap as pymmap
+import os
+
+cdef extern from "sys/mman.h":
+    cdef char *MAP_FAILED
+
+cdef _raise_oserror(message=''):
+    if message:
+        message += ': '
+    message += os.strerror(errno)
+    return OSError(errno, message)
+
+cdef class mmap:
+    cdef char *ptr
+    cdef size_t len
+
+    def __cinit__(self, int fd, size_t length, int access=pymmap.ACCESS_READ):
+        cdef stat.struct_stat st
+        # Only support a subset of options for now
+        if access != pymmap.ACCESS_READ:
+            raise RuntimeError('access %s is unsupported' % access)
+
+        # If length is 0, read size from file
+        if length == 0:
+            r = stat.fstat(fd, &st)
+            if r != 0:
+                _raise_oserror('fstat failed')
+            length = st.st_size
+
+        self.ptr = <char*>mman.mmap(NULL, length, mman.PROT_READ,
+                                    mman.MAP_SHARED, fd, 0)
+        if self.ptr == MAP_FAILED:
+            _raise_oserror('mmap failed')
+
+        self.len = length
+
+    # This method should be called "unmap". But it's to be compatible with
+    # the Python API.
+    cpdef close(self):
+        if self.ptr == NULL:
+            return
+
+        r = mman.munmap(self.ptr, self.len)
+        if r != 0:
+            _raise_oserror('munmap failed')
+        self.ptr = NULL
+
+    def __getslice__(self, Py_ssize_t i, Py_ssize_t j):
+        if self.ptr == NULL:
+            raise RuntimeError('mmap closed')
+        if j > self.len:
+            j = self.len
+        return PyBytes_FromStringAndSize(self.ptr + i, j - i)
+
+    def __len__(self):
+        return self.len
+
+    def __dealloc__(self):
+        self.close()
diff --git a/remotefilelog/basepack.py b/remotefilelog/basepack.py
--- a/remotefilelog/basepack.py
+++ b/remotefilelog/basepack.py
@@ -9,6 +9,14 @@
 
 from . import shallowutil
 
+mmapfunc = mmap.mmap
+try:
+    from hgext3rd import extmmap
+except ImportError:
+    pass
+else:
+    mmapfunc = extmmap.mmap
+
 osutil = policy.importmod(r'osutil')
 
 # The pack version supported by this implementation. This will need to be
@@ -310,8 +318,9 @@
         if self.VERSION == 0:
             return self.indexsize
         else:
-            nodecount = struct.unpack_from('!Q', self._index,
-                                           self.params.indexstart - 8)[0]
+            offset = self.params.indexstart - 8
+            nodecount = struct.unpack_from('!Q',
+                                           self._index[offset:offset + 8])[0]
             return self.params.indexstart + nodecount * self.INDEXENTRYLENGTH
 
     def freememory(self):
@@ -328,10 +337,10 @@
         # TODO: use an opener/vfs to access these paths
         with open(self.indexpath, PACKOPENMODE) as indexfp:
             # memory-map the file, size 0 means whole file
-            self._index = mmap.mmap(indexfp.fileno(), 0,
-                                    access=mmap.ACCESS_READ)
+            self._index = mmapfunc(indexfp.fileno(), 0,
+                                   access=mmap.ACCESS_READ)
         with open(self.packpath, PACKOPENMODE) as datafp:
-            self._data = mmap.mmap(datafp.fileno(), 0, access=mmap.ACCESS_READ)
+            self._data = mmapfunc(datafp.fileno(), 0, access=mmap.ACCESS_READ)
 
         self._pagedin = 0
         return True
diff --git a/remotefilelog/datapack.py b/remotefilelog/datapack.py
--- a/remotefilelog/datapack.py
+++ b/remotefilelog/datapack.py
@@ -277,7 +277,7 @@
 
             if self.VERSION == 1:
                 # <4 byte len> + <metadata-list>
-                metalen = struct.unpack_from('!I', data, offset)[0]
+                metalen = struct.unpack('!I', data[offset:offset + 4])[0]
                 offset += 4 + metalen
 
             yield (filename, node, deltabase, uncompressedlen)
diff --git a/setup.py b/setup.py
--- a/setup.py
+++ b/setup.py
@@ -296,6 +296,14 @@
                 ] + cflags),
             ),
         ],
+        'extmmap': [
+            Extension('hgext3rd.extmmap',
+                sources=['hgext3rd/extmmap.pyx'],
+                extra_compile_args=filter(None, [
+                    STDC99, WALL, WEXTRA, WCONVERSION, PEDANTIC,
+                ]),
+            ),
+        ],
         'linelog' : [
             Extension('linelog',
                 sources=['linelog/pyext/linelog.pyx'],
@@ -354,6 +362,7 @@
     cythonmodules = ['linelog']
 else:
     cythonmodules = [
+        'extmmap',
         'linelog',
         'patchrmdir',
         'traceprof',
diff --git a/tests/test-remotefilelog-datapack.py b/tests/test-remotefilelog-datapack.py
--- a/tests/test-remotefilelog-datapack.py
+++ b/tests/test-remotefilelog-datapack.py
@@ -38,6 +38,7 @@
 class datapacktestsbase(object):
     def __init__(self, datapackreader, paramsavailable):
         self.datapackreader = datapackreader
+        self.iscdatapack = not paramsavailable
         self.paramsavailable = paramsavailable
 
     def setUp(self):
@@ -275,11 +276,11 @@
     def testPacksCache(self):
         """Test that we remember the most recent packs while fetching the delta
         chain."""
-
         packdir = self.makeTempDir()
         deltachains = []
 
-        numpacks = 10
+        # Ensures that we are not keeping everything in the cache.
+        numpacks = datapackstore.DEFAULTCACHESIZE * 2
         revisionsperpack = 100
 
         for i in range(numpacks):
@@ -298,11 +299,7 @@
             self.createPack(chain, packdir)
             deltachains.append(chain)
 
-        class testdatapackstore(datapackstore):
-            # Ensures that we are not keeping everything in the cache.
-            DEFAULTCACHESIZE = numpacks / 2
-
-        store = testdatapackstore(mercurial.ui.ui(), packdir)
+        store = datapackstore(mercurial.ui.ui(), packdir, self.iscdatapack)
 
         random.shuffle(deltachains)
         for randomchain in deltachains: