diff --git a/cmds/idmap2/idmap2d/Idmap2Service.cpp b/cmds/idmap2/idmap2d/Idmap2Service.cpp
index e55ea6c..75fc7f7 100644
--- a/cmds/idmap2/idmap2d/Idmap2Service.cpp
+++ b/cmds/idmap2/idmap2d/Idmap2Service.cpp
@@ -149,15 +149,21 @@
     return error(idmap.GetErrorMessage());
   }
 
+  // idmap files are mapped with mmap in libandroidfw. Deleting and recreating the idmap guarantees
+  // that existing memory maps will continue to be valid and unaffected.
+  unlink(idmap_path.c_str());
+
   umask(kIdmapFilePermissionMask);
   std::ofstream fout(idmap_path);
   if (fout.fail()) {
     return error("failed to open idmap path " + idmap_path);
   }
+
   BinaryStreamVisitor visitor(fout);
   (*idmap)->accept(&visitor);
   fout.close();
   if (fout.fail()) {
+    unlink(idmap_path.c_str());
     return error("failed to write to idmap path " + idmap_path);
   }
 
diff --git a/core/java/android/app/ResourcesManager.java b/core/java/android/app/ResourcesManager.java
index d00366b..2e2612b 100644
--- a/core/java/android/app/ResourcesManager.java
+++ b/core/java/android/app/ResourcesManager.java
@@ -344,7 +344,7 @@
         ApkAssets apkAssets = null;
         if (mLoadedApkAssets != null) {
             apkAssets = mLoadedApkAssets.get(newKey);
-            if (apkAssets != null) {
+            if (apkAssets != null && apkAssets.isUpToDate()) {
                 return apkAssets;
             }
         }
@@ -353,7 +353,7 @@
         final WeakReference<ApkAssets> apkAssetsRef = mCachedApkAssets.get(newKey);
         if (apkAssetsRef != null) {
             apkAssets = apkAssetsRef.get();
-            if (apkAssets != null) {
+            if (apkAssets != null && apkAssets.isUpToDate()) {
                 if (mLoadedApkAssets != null) {
                     mLoadedApkAssets.put(newKey, apkAssets);
                 }
diff --git a/libs/androidfw/ApkAssets.cpp b/libs/androidfw/ApkAssets.cpp
index 918e7af..05f4d6b 100644
--- a/libs/androidfw/ApkAssets.cpp
+++ b/libs/androidfw/ApkAssets.cpp
@@ -385,7 +385,7 @@
   const StringPiece idmap_data(
       reinterpret_cast<const char*>(idmap_asset->getBuffer(true /*wordAligned*/)),
       static_cast<size_t>(idmap_asset->getLength()));
-  std::unique_ptr<const LoadedIdmap> loaded_idmap = LoadedIdmap::Load(idmap_data);
+  std::unique_ptr<const LoadedIdmap> loaded_idmap = LoadedIdmap::Load(idmap_path, idmap_data);
   if (loaded_idmap == nullptr) {
     LOG(ERROR) << "failed to load IDMAP " << idmap_path;
     return {};
@@ -538,8 +538,9 @@
     // Loaders are invalidated by the app, not the system, so assume they are up to date.
     return true;
   }
+  return (!loaded_idmap_ || loaded_idmap_->IsUpToDate()) &&
+      last_mod_time_ == getFileModDate(path_.c_str());
 
-  return last_mod_time_ == getFileModDate(path_.c_str());
 }
 
 }  // namespace android
diff --git a/libs/androidfw/Idmap.cpp b/libs/androidfw/Idmap.cpp
index 0b2fd9e..eb6ee95 100644
--- a/libs/androidfw/Idmap.cpp
+++ b/libs/androidfw/Idmap.cpp
@@ -20,6 +20,7 @@
 
 #include "android-base/logging.h"
 #include "android-base/stringprintf.h"
+#include "androidfw/misc.h"
 #include "androidfw/ResourceTypes.h"
 #include "androidfw/Util.h"
 #include "utils/ByteOrder.h"
@@ -192,7 +193,9 @@
   return true;
 }
 
-LoadedIdmap::LoadedIdmap(const Idmap_header* header,
+LoadedIdmap::LoadedIdmap(std::string&& idmap_path,
+                         const time_t last_mod_time,
+                         const Idmap_header* header,
                          const Idmap_data_header* data_header,
                          const Idmap_target_entry* target_entries,
                          const Idmap_overlay_entry* overlay_entries,
@@ -201,7 +204,9 @@
        data_header_(data_header),
        target_entries_(target_entries),
        overlay_entries_(overlay_entries),
-       string_pool_(string_pool) {
+       string_pool_(string_pool),
+       idmap_path_(std::move(idmap_path)),
+       idmap_last_mod_time_(last_mod_time) {
 
   size_t length = strnlen(reinterpret_cast<const char*>(header_->overlay_path),
                           arraysize(header_->overlay_path));
@@ -212,7 +217,8 @@
   target_apk_path_.assign(reinterpret_cast<const char*>(header_->target_path), length);
 }
 
-std::unique_ptr<const LoadedIdmap> LoadedIdmap::Load(const StringPiece& idmap_data) {
+std::unique_ptr<const LoadedIdmap> LoadedIdmap::Load(const StringPiece& idmap_path,
+                                                     const StringPiece& idmap_data) {
   ATRACE_CALL();
   if (!IsValidIdmapHeader(idmap_data)) {
     return {};
@@ -275,10 +281,14 @@
 
   // Can't use make_unique because LoadedIdmap constructor is private.
   std::unique_ptr<LoadedIdmap> loaded_idmap = std::unique_ptr<LoadedIdmap>(
-      new LoadedIdmap(header, data_header, target_entries, overlay_entries,
-                      idmap_string_pool.release()));
+      new LoadedIdmap(idmap_path.to_string(), getFileModDate(idmap_path.data()), header,
+                      data_header, target_entries, overlay_entries, idmap_string_pool.release()));
 
   return std::move(loaded_idmap);
 }
 
+bool LoadedIdmap::IsUpToDate() const {
+  return idmap_last_mod_time_ == getFileModDate(idmap_path_.c_str());
+}
+
 }  // namespace android
diff --git a/libs/androidfw/include/androidfw/Idmap.h b/libs/androidfw/include/androidfw/Idmap.h
index ccb57f3..ecc1ce6 100644
--- a/libs/androidfw/include/androidfw/Idmap.h
+++ b/libs/androidfw/include/androidfw/Idmap.h
@@ -142,7 +142,13 @@
 class LoadedIdmap {
  public:
   // Loads an IDMAP from a chunk of memory. Returns nullptr if the IDMAP data was malformed.
-  static std::unique_ptr<const LoadedIdmap> Load(const StringPiece& idmap_data);
+  static std::unique_ptr<const LoadedIdmap> Load(const StringPiece& idmap_path,
+                                                 const StringPiece& idmap_data);
+
+  // Returns the path to the IDMAP.
+  inline const std::string& IdmapPath() const {
+    return idmap_path_;
+  }
 
   // Returns the path to the RRO (Runtime Resource Overlay) APK for which this IDMAP was generated.
   inline const std::string& OverlayApkPath() const {
@@ -167,6 +173,10 @@
     return OverlayDynamicRefTable(data_header_, overlay_entries_, target_assigned_package_id);
   }
 
+  // Returns whether the idmap file on disk has not been modified since the construction of this
+  // LoadedIdmap.
+  bool IsUpToDate() const;
+
  protected:
   // Exposed as protected so that tests can subclass and mock this class out.
   LoadedIdmap() = default;
@@ -177,13 +187,17 @@
   const Idmap_overlay_entry* overlay_entries_;
   const std::unique_ptr<ResStringPool> string_pool_;
 
+  const std::string idmap_path_;
   std::string overlay_apk_path_;
   std::string target_apk_path_;
+  const time_t idmap_last_mod_time_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(LoadedIdmap);
 
-  explicit LoadedIdmap(const Idmap_header* header,
+  explicit LoadedIdmap(std::string&& idmap_path,
+                       time_t last_mod_time,
+                       const Idmap_header* header,
                        const Idmap_data_header* data_header,
                        const Idmap_target_entry* target_entries,
                        const Idmap_overlay_entry* overlay_entries,
diff --git a/libs/androidfw/tests/Idmap_test.cpp b/libs/androidfw/tests/Idmap_test.cpp
index 41ba637..7aa0dbb 100644
--- a/libs/androidfw/tests/Idmap_test.cpp
+++ b/libs/androidfw/tests/Idmap_test.cpp
@@ -38,7 +38,7 @@
  protected:
   void SetUp() override {
     // Move to the test data directory so the idmap can locate the overlay APK.
-    std::string original_path = base::GetExecutableDirectory();
+    original_path = base::GetExecutableDirectory();
     chdir(GetTestDataPath().c_str());
 
     system_assets_ = ApkAssets::Load("system/system.apk");
@@ -49,10 +49,14 @@
 
     overlayable_assets_ = ApkAssets::Load("overlayable/overlayable.apk");
     ASSERT_NE(nullptr, overlayable_assets_);
+  }
+
+  void TearDown() override {
     chdir(original_path.c_str());
   }
 
  protected:
+  std::string original_path;
   std::unique_ptr<const ApkAssets> system_assets_;
   std::unique_ptr<const ApkAssets> overlay_assets_;
   std::unique_ptr<const ApkAssets> overlayable_assets_;
@@ -221,8 +225,7 @@
 
 TEST_F(IdmapTest, OverlayLoaderInterop) {
   std::string contents;
-  auto loader_assets = ApkAssets::LoadTable(GetTestDataPath() + "/loader/resources.arsc",
-                                            PROPERTY_LOADER);
+  auto loader_assets = ApkAssets::LoadTable("loader/resources.arsc", PROPERTY_LOADER);
 
   AssetManager2 asset_manager;
   asset_manager.SetApkAssets({overlayable_assets_.get(), loader_assets.get(),
@@ -241,4 +244,25 @@
   ASSERT_EQ(GetStringFromApkAssets(asset_manager, val, cookie), "loader");
 }
 
+TEST_F(IdmapTest, OverlayAssetsIsUpToDate) {
+  std::string idmap_contents;
+  ASSERT_TRUE(base::ReadFileToString("overlay/overlay.idmap", &idmap_contents));
+
+  TemporaryFile temp_file;
+  ASSERT_TRUE(base::WriteStringToFile(idmap_contents, temp_file.path));
+
+  auto apk_assets = ApkAssets::LoadOverlay(temp_file.path);
+  ASSERT_NE(nullptr, apk_assets);
+  ASSERT_TRUE(apk_assets->IsUpToDate());
+
+  unlink(temp_file.path);
+  ASSERT_FALSE(apk_assets->IsUpToDate());
+  sleep(2);
+
+  base::WriteStringToFile("hello", temp_file.path);
+  sleep(2);
+
+  ASSERT_FALSE(apk_assets->IsUpToDate());
+}
+
 }  // namespace
diff --git a/services/core/java/com/android/server/om/IdmapManager.java b/services/core/java/com/android/server/om/IdmapManager.java
index 43fc7ed..90c85ad 100644
--- a/services/core/java/com/android/server/om/IdmapManager.java
+++ b/services/core/java/com/android/server/om/IdmapManager.java
@@ -63,20 +63,23 @@
         mIdmapDaemon = IdmapDaemon.getInstance();
     }
 
+    /**
+     * Creates the idmap for the target/overlay combination and returns whether the idmap file was
+     * modified.
+     */
     boolean createIdmap(@NonNull final PackageInfo targetPackage,
             @NonNull final PackageInfo overlayPackage, int userId) {
         if (DEBUG) {
             Slog.d(TAG, "create idmap for " + targetPackage.packageName + " and "
                     + overlayPackage.packageName);
         }
-        final int sharedGid = UserHandle.getSharedAppGid(targetPackage.applicationInfo.uid);
         final String targetPath = targetPackage.applicationInfo.getBaseCodePath();
         final String overlayPath = overlayPackage.applicationInfo.getBaseCodePath();
         try {
             int policies = calculateFulfilledPolicies(targetPackage, overlayPackage, userId);
             boolean enforce = enforceOverlayable(overlayPackage);
             if (mIdmapDaemon.verifyIdmap(targetPath, overlayPath, policies, enforce, userId)) {
-                return true;
+                return false;
             }
             return mIdmapDaemon.createIdmap(targetPath, overlayPath, policies,
                     enforce, userId) != null;
diff --git a/services/core/java/com/android/server/om/OverlayManagerServiceImpl.java b/services/core/java/com/android/server/om/OverlayManagerServiceImpl.java
index d108e76..05a4a38 100644
--- a/services/core/java/com/android/server/om/OverlayManagerServiceImpl.java
+++ b/services/core/java/com/android/server/om/OverlayManagerServiceImpl.java
@@ -700,14 +700,15 @@
         final PackageInfo overlayPackage = mPackageManager.getPackageInfo(overlayPackageName,
                 userId);
 
-        // Immutable RROs targeting to "android", ie framework-res.apk, are handled by native layers.
+        // Immutable RROs targeting to "android", ie framework-res.apk, are handled by native
+        // layers.
+        boolean modified = false;
         if (targetPackage != null && overlayPackage != null
                 && !("android".equals(targetPackageName)
                     && !isPackageConfiguredMutable(overlayPackageName))) {
-            mIdmapManager.createIdmap(targetPackage, overlayPackage, userId);
+            modified |= mIdmapManager.createIdmap(targetPackage, overlayPackage, userId);
         }
 
-        boolean modified = false;
         if (overlayPackage != null) {
             modified |= mSettings.setBaseCodePath(overlayPackageName, userId,
                     overlayPackage.applicationInfo.getBaseCodePath());