diff --git a/clang/include/clang/Serialization/ASTWriter.h b/clang/include/clang/Serialization/ASTWriter.h
index 3ed9803fa3745..bd310b6c7a5cd 100644
--- a/clang/include/clang/Serialization/ASTWriter.h
+++ b/clang/include/clang/Serialization/ASTWriter.h
@@ -846,7 +846,7 @@ class ASTWriter : public ASTDeserializationListener,
 /// AST and semantic-analysis consumer that generates a
 /// precompiled header from the parsed source code.
 class PCHGenerator : public SemaConsumer {
-  const Preprocessor &PP;
+  Preprocessor &PP;
   std::string OutputFile;
   std::string isysroot;
   Sema *SemaPtr;
@@ -867,11 +867,12 @@ class PCHGenerator : public SemaConsumer {
   DiagnosticsEngine &getDiagnostics() const {
     return SemaPtr->getDiagnostics();
   }
+  Preprocessor &getPreprocessor() { return PP; }
 
   virtual Module *getEmittingModule(ASTContext &Ctx);
 
 public:
-  PCHGenerator(const Preprocessor &PP, InMemoryModuleCache &ModuleCache,
+  PCHGenerator(Preprocessor &PP, InMemoryModuleCache &ModuleCache,
                StringRef OutputFile, StringRef isysroot,
                std::shared_ptr<PCHBuffer> Buffer,
                ArrayRef<std::shared_ptr<ModuleFileExtension>> Extensions,
@@ -893,7 +894,7 @@ class ReducedBMIGenerator : public PCHGenerator {
   virtual Module *getEmittingModule(ASTContext &Ctx) override;
 
 public:
-  ReducedBMIGenerator(const Preprocessor &PP, InMemoryModuleCache &ModuleCache,
+  ReducedBMIGenerator(Preprocessor &PP, InMemoryModuleCache &ModuleCache,
                       StringRef OutputFile);
 
   void HandleTranslationUnit(ASTContext &Ctx) override;
diff --git a/clang/lib/Frontend/PrecompiledPreamble.cpp b/clang/lib/Frontend/PrecompiledPreamble.cpp
index 9b0ef30a14121..fdf05c3613c95 100644
--- a/clang/lib/Frontend/PrecompiledPreamble.cpp
+++ b/clang/lib/Frontend/PrecompiledPreamble.cpp
@@ -290,8 +290,7 @@ class PrecompilePreambleAction : public ASTFrontendAction {
 
 class PrecompilePreambleConsumer : public PCHGenerator {
 public:
-  PrecompilePreambleConsumer(PrecompilePreambleAction &Action,
-                             const Preprocessor &PP,
+  PrecompilePreambleConsumer(PrecompilePreambleAction &Action, Preprocessor &PP,
                              InMemoryModuleCache &ModuleCache,
                              StringRef isysroot,
                              std::shared_ptr<PCHBuffer> Buffer)
diff --git a/clang/lib/Serialization/GeneratePCH.cpp b/clang/lib/Serialization/GeneratePCH.cpp
index f54db36d4a019..fa712264d54a9 100644
--- a/clang/lib/Serialization/GeneratePCH.cpp
+++ b/clang/lib/Serialization/GeneratePCH.cpp
@@ -14,6 +14,7 @@
 #include "clang/AST/ASTContext.h"
 #include "clang/Frontend/FrontendDiagnostic.h"
 #include "clang/Lex/HeaderSearch.h"
+#include "clang/Lex/HeaderSearchOptions.h"
 #include "clang/Lex/Preprocessor.h"
 #include "clang/Sema/SemaConsumer.h"
 #include "clang/Serialization/ASTReader.h"
@@ -23,8 +24,8 @@
 using namespace clang;
 
 PCHGenerator::PCHGenerator(
-    const Preprocessor &PP, InMemoryModuleCache &ModuleCache,
-    StringRef OutputFile, StringRef isysroot, std::shared_ptr<PCHBuffer> Buffer,
+    Preprocessor &PP, InMemoryModuleCache &ModuleCache, StringRef OutputFile,
+    StringRef isysroot, std::shared_ptr<PCHBuffer> Buffer,
     ArrayRef<std::shared_ptr<ModuleFileExtension>> Extensions,
     bool AllowASTWithErrors, bool IncludeTimestamps,
     bool BuildingImplicitModule, bool ShouldCacheASTInMemory,
@@ -88,7 +89,7 @@ ASTDeserializationListener *PCHGenerator::GetASTDeserializationListener() {
   return &Writer;
 }
 
-ReducedBMIGenerator::ReducedBMIGenerator(const Preprocessor &PP,
+ReducedBMIGenerator::ReducedBMIGenerator(Preprocessor &PP,
                                          InMemoryModuleCache &ModuleCache,
                                          StringRef OutputFile)
     : PCHGenerator(
@@ -107,6 +108,18 @@ Module *ReducedBMIGenerator::getEmittingModule(ASTContext &Ctx) {
 }
 
 void ReducedBMIGenerator::HandleTranslationUnit(ASTContext &Ctx) {
+  // We need to do this to make sure the size of reduced BMI not to be larger
+  // than full BMI.
+  //
+  // FIMXE: We'd better to wrap such options to a new class ASTWriterOptions
+  // since this is not about searching header really.
+  // FIXME2: We'd better to move the class writing full BMI with reduced BMI.
+  HeaderSearchOptions &HSOpts =
+      getPreprocessor().getHeaderSearchInfo().getHeaderSearchOpts();
+  HSOpts.ModulesSkipDiagnosticOptions = true;
+  HSOpts.ModulesSkipHeaderSearchPaths = true;
+  HSOpts.ModulesSkipPragmaDiagnosticMappings = true;
+
   PCHGenerator::HandleTranslationUnit(Ctx);
 
   if (!isComplete())
diff --git a/clang/test/Modules/reduced-bmi-size.cppm b/clang/test/Modules/reduced-bmi-size.cppm
new file mode 100644
index 0000000000000..664f45f5c6a5a
--- /dev/null
+++ b/clang/test/Modules/reduced-bmi-size.cppm
@@ -0,0 +1,16 @@
+// Ensure that the size of the reduced BMI is not larger than the full BMI
+// in the most simple case. 
+
+// This test requires linux commands.
+// REQUIRES: system-linux
+
+// RUN: rm -fr %t
+// RUN: mkdir %t
+//
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %s -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 -emit-reduced-module-interface %s -o %t/a.reduced.pcm
+//
+// %s implies the current source file. So we can't use it directly.
+// RUN: [ $(stat -c%\s "%t/a.pcm") -le $(stat -c%\s "%t/a.reduced.pcm") ]
+
+export module a;