diff --git a/src/core/IronPython.Modules/mmap.cs b/src/core/IronPython.Modules/mmap.cs
index 56f99230b..d33f997c3 100644
--- a/src/core/IronPython.Modules/mmap.cs
+++ b/src/core/IronPython.Modules/mmap.cs
@@ -920,6 +920,8 @@ public void resize(long newsize) {
                         throw PythonOps.ValueError("new size out of range");
                     }
 
+                    long capacity = checked(_offset + newsize);
+
                     if (_handle is not null
                         && (RuntimeInformation.IsOSPlatform(OSPlatform.OSX) || RuntimeInformation.IsOSPlatform(OSPlatform.Linux))) {
                         // resize on Posix platforms
@@ -941,14 +943,14 @@ public void resize(long newsize) {
 
                             // Resize the underlying file as needed.
                             int fd = unchecked((int)_handle.DangerousGetHandle());
-                            PythonNT.ftruncateUnix(fd, newsize);
+                            PythonNT.ftruncateUnix(fd, capacity);
 
 #if NET8_0_OR_GREATER
-                            _file = MemoryMappedFile.CreateFromFile(_handle, _mapName, newsize, _fileAccess, HandleInheritability.None, leaveOpen: true);
+                            _file = MemoryMappedFile.CreateFromFile(_handle, _mapName, capacity, _fileAccess, HandleInheritability.None, leaveOpen: true);
 #else
                             _sourceStream?.Dispose();
                             _sourceStream = new FileStream(new SafeFileHandle((IntPtr)fd, ownsHandle: false), FileAccess.ReadWrite);
-                            _file = CreateFromFile(_sourceStream, _mapName, newsize, _fileAccess, HandleInheritability.None, leaveOpen: true);
+                            _file = CreateFromFile(_sourceStream, _mapName, capacity, _fileAccess, HandleInheritability.None, leaveOpen: true);
 #endif
                             _view = _file.CreateViewAccessor(_offset, newsize, _fileAccess);
                             return;
@@ -968,8 +970,6 @@ public void resize(long newsize) {
                         return;
                     }
 
-                    long capacity = checked(_offset + newsize);
-
                     try {
                         if (newsize == 0) {
                             // resizing to an empty mapped region is not allowed
@@ -1401,17 +1401,18 @@ private struct SYSTEM_INFO {
             internal short wProcessorRevision;
         }
 
+        [SupportedOSPlatform("windows")]
         [DllImport("kernel32", SetLastError = true)]
         private static extern void GetSystemInfo(ref SYSTEM_INFO lpSystemInfo);
 
         private static int GetAllocationGranularity() {
-            try {
+            if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows)) {
                 return GetAllocationGranularityWorker();
-            } catch {
-                return System.Environment.SystemPageSize;
             }
+            return System.Environment.SystemPageSize;
         }
 
+        [SupportedOSPlatform("windows")]
         [MethodImpl(MethodImplOptions.NoInlining)]
         private static int GetAllocationGranularityWorker() {
             SYSTEM_INFO info = new SYSTEM_INFO();
diff --git a/src/core/IronPython.StdLib/lib/test/test_mmap.py b/src/core/IronPython.StdLib/lib/test/test_mmap.py
index 7882be8ca..d21d25578 100644
--- a/src/core/IronPython.StdLib/lib/test/test_mmap.py
+++ b/src/core/IronPython.StdLib/lib/test/test_mmap.py
@@ -102,7 +102,7 @@ def test_basic(self):
             # resize() not supported
             # No messages are printed, since the output of this test suite
             # would then be different across platforms.
-            pass
+            raise # ironpython: all our runners currently support resize
         else:
             # resize() is supported
             self.assertEqual(len(m), 512)
@@ -168,7 +168,7 @@ def test_access_parameter(self):
             try:
                 m.resize(2*mapsize)
             except SystemError:   # resize is not universally supported
-                pass
+                raise # ironpython: all our runners currently support resize
             except TypeError:
                 pass
             else:
@@ -539,7 +539,7 @@ def test_offset (self):
             try:
                 m.resize(512)
             except SystemError:
-                pass
+                raise # ironpython: all our runners currently support resize
             else:
                 # resize() is supported
                 self.assertEqual(len(m), 512)