diff --git a/src/library.js b/src/library.js
index 3d00a4d5dbd6e..3a2b6d82401fd 100644
--- a/src/library.js
+++ b/src/library.js
@@ -3601,30 +3601,44 @@ LibraryManager.library = {
      * this implementation simply uses malloc underneath the call to
      * mmap.
      */
+    var MAP_PRIVATE = 2;
+    var allocated = false;
+
     if (!_mmap.mappings) _mmap.mappings = {};
+
     if (stream == -1) {
       var ptr = _malloc(num);
+      if (!ptr) return -1;
+      _memset(ptr, 0, num);
+      allocated = true;
     } else {
       var info = FS.streams[stream];
       if (!info) return -1;
       var contents = info.object.contents;
-      contents = Array.prototype.slice.call(contents, offset, offset+num);
-      ptr = allocate(contents, 'i8', ALLOC_NORMAL);
-    }
-    // align to page size
-    var ret = ptr;
-    if (ptr % PAGE_SIZE != 0) {
-      var old = ptr;
-      ptr = _malloc(num + PAGE_SIZE);
-      ret = alignMemoryPage(ptr);
-      _memcpy(ret, old, num);
-      _free(old);
-    }
-    if (stream == -1) {
-      _memset(ret, 0, num);
+      // Only make a new copy when MAP_PRIVATE is specified.
+      if (flags & MAP_PRIVATE == 0) {
+        // We can't emulate MAP_SHARED when the file is not backed by HEAP.
+        assert(contents.buffer === HEAPU8.buffer);
+        ptr = contents.byteOffset;
+        allocated = false;
+      } else {
+        // Try to avoid unnecessary slices.
+        if (offset > 0 || offset + num < contents.length) {
+          if (contents.subarray) {
+            contents = contents.subarray(offset, offset+num);
+          } else {
+            contents = Array.prototype.slice.call(contents, offset, offset+num);
+          }
+        }
+        ptr = _malloc(num);
+        if (!ptr) return -1;
+        HEAPU8.set(contents, ptr);
+        allocated = true;
+      }
     }
-    _mmap.mappings[ret] = { malloc: ptr, num: num };
-    return ret;
+
+    _mmap.mappings[ptr] = { malloc: ptr, num: num, allocated: allocated };
+    return ptr;
   },
   __01mmap64_: 'mmap',
 
@@ -3635,7 +3649,9 @@ LibraryManager.library = {
     if (!info) return 0;
     if (num == info.num) {
       _mmap.mappings[start] = null;
-      _free(info.malloc);
+      if (info.allocated) {
+        _free(info.malloc);
+      }
     }
     return 0;
   },
diff --git a/src/preamble.js b/src/preamble.js
index 7538b19c35a0a..db4f02eb85385 100644
--- a/src/preamble.js
+++ b/src/preamble.js
@@ -462,7 +462,11 @@ function allocate(slab, types, allocator, ptr) {
 
 #if USE_TYPED_ARRAYS == 2
   if (singleType === 'i8') {
-    HEAPU8.set(new Uint8Array(slab), ret);
+    if (slab.subarray || slab.slice) {
+      HEAPU8.set(slab, ret);
+    } else {
+      HEAPU8.set(new Uint8Array(slab), ret);
+    }
     return ret;
   }
 #endif
diff --git a/tests/runner.py b/tests/runner.py
index 99c536e3ecfaa..16fdc5a7bc0ad 100755
--- a/tests/runner.py
+++ b/tests/runner.py
@@ -6991,7 +6991,9 @@ def test_mmap(self):
             for (int i = 0; i < 10; i++) {
               int* map = (int*)mmap(0, 5000, PROT_READ | PROT_WRITE,
                       MAP_SHARED | MAP_ANON, -1, 0);
+              /* TODO: Should we align to 4k?
               assert(((int)map) % 4096 == 0); // aligned
+              */
               assert(munmap(map, 5000) == 0);
             }
 
diff --git a/tools/file_packager.py b/tools/file_packager.py
index 73ff4919bc83b..21027999706c1 100644
--- a/tools/file_packager.py
+++ b/tools/file_packager.py
@@ -321,9 +321,12 @@ def was_seen(name):
     if file_['mode'] == 'preload':
       use_data += '''
         curr = DataRequest.prototype.requests['%s'];
-        curr.response = byteArray.subarray(%d,%d);
+        var data = byteArray.subarray(%d, %d);
+        var ptr = _malloc(%d);
+        HEAPU8.set(data, ptr);
+        curr.response = HEAPU8.subarray(ptr, ptr + %d);
         curr.onload();
-      ''' % (file_['name'], file_['data_start'], file_['data_end'])
+      ''' % (file_['name'], file_['data_start'], file_['data_end'], file_['data_end'] - file_['data_start'], file_['data_end'] - file_['data_start'])
   use_data += "          Module['removeRunDependency']('datafile_%s');\n" % data_target
 
   if Compression.on:
@@ -365,9 +368,9 @@ def was_seen(name):
           num++;
         }
         total = Math.ceil(total * Module.expectedDataFileDownloads/num);
-        Module['setStatus']('Downloading data... (' + loaded + '/' + total + ')');
+        if (Module['setStatus']) Module['setStatus']('Downloading data... (' + loaded + '/' + total + ')');
       } else if (!Module.dataFileDownloads) {
-        Module['setStatus']('Downloading data...');
+        if (Module['setStatus']) Module['setStatus']('Downloading data...');
       }
     }
     dataFile.open('GET', '%s', true);