diff --git a/bump_pydantic/main.py b/bump_pydantic/main.py
index 63ee085..dc6a7f8 100644
--- a/bump_pydantic/main.py
+++ b/bump_pydantic/main.py
@@ -1,4 +1,5 @@
 import difflib
+import multiprocessing
 import os
 import time
 from pathlib import Path
@@ -69,47 +70,51 @@ def main(
     codemods = gather_codemods()
 
     # TODO: We can run this in parallel - batch it into files / cores.
-    # with ProcessPoolExecutor():
-    #     cpu_count = multiprocessing.cpu_count()
-    #     batch_size = len(files) // cpu_count + 1
-
-    #     batches = [files[i : i + batch_size] for i in range(0, len(files), batch_size)]
-    #     print(batches)
-
-    for codemod in codemods:
-        for filename in files:
-            module_and_package = calculate_module_and_package(str(package), filename)
-            context = CodemodContext(
-                metadata_manager=metadata_manager,
-                filename=filename,
-                full_module_name=module_and_package.name,
-                full_package_name=module_and_package.package,
-            )
-            context.scratch.update(scratch)
-
-            transformer = codemod(context=context)
-
-            old_code = Path(filename).read_text()
-            input_tree = cst.parse_module(old_code)
-            output_tree = transformer.transform_module(input_tree)
-
-            input_code = input_tree.code
-            output_code = output_tree.code
-
-            if input_code != output_code:
+    with multiprocessing.Pool():
+        cpu_count = multiprocessing.cpu_count()
+        batch_size = len(files) // cpu_count + 1
+
+        [files[i : i + batch_size] for i in range(0, len(files), batch_size)]
+
+    for filename in files:
+        module_and_package = calculate_module_and_package(str(package), filename)
+        context = CodemodContext(
+            metadata_manager=metadata_manager,
+            filename=filename,
+            full_module_name=module_and_package.name,
+            full_package_name=module_and_package.package,
+        )
+        context.scratch.update(scratch)
+
+        file_path = Path(filename)
+        with file_path.open("r+") as fp:
+            code = fp.read()
+            fp.seek(0)
+
+            input_code = str(code)
+
+            for codemod in codemods:
+                transformer = codemod(context=context)
+
+                input_tree = cst.parse_module(input_code)
+                output_tree = transformer.transform_module(input_tree)
+
+                input_code = output_tree.code
+
+            if code != input_code:
                 if diff:
                     color_diff(
                         console=console,
                         lines=difflib.unified_diff(
+                            code.splitlines(keepends=True),
                             input_code.splitlines(keepends=True),
-                            output_code.splitlines(keepends=True),
                             fromfile=filename,
                             tofile=filename,
                         ),
                     )
                 else:
-                    with open(filename, "w") as fp:
-                        fp.write(output_tree.code)
+                    fp.write(input_code)
+                    fp.truncate()
 
     modified = [Path(f) for f in files if os.stat(f).st_mtime > start_time]
     if modified: