Update from https://crrev.com/328418

base/android/java/src/org/chromium/base/library_loader/LibraryLoader.java

Index: base/android/java/src/org/chromium/base/library_loader/LibraryLoader.java
diff --git a/base/android/java/src/org/chromium/base/library_loader/LibraryLoader.java b/base/android/java/src/org/chromium/base/library_loader/LibraryLoader.java
index 7dc015dc8f9ecb7c420e55046b2351646fb9fd2f..9dc686572d0bb4a5f85b2b74c8fa5d012c6fb480 100644
--- a/base/android/java/src/org/chromium/base/library_loader/LibraryLoader.java
+++ b/base/android/java/src/org/chromium/base/library_loader/LibraryLoader.java
@@ -5,7 +5,6 @@
 package org.chromium.base.library_loader;
 
 import android.content.Context;
-import android.content.pm.ApplicationInfo;
 import android.os.AsyncTask;
 import android.os.SystemClock;
 import android.util.Log;
@@ -15,14 +14,8 @@ import org.chromium.base.CommandLine;
 import org.chromium.base.JNINamespace;
 import org.chromium.base.TraceEvent;
 import org.chromium.base.VisibleForTesting;
+import org.chromium.base.metrics.RecordHistogram;
 
-import java.io.File;
-import java.io.FileNotFoundException;
-import java.io.IOException;
-import java.io.RandomAccessFile;
-import java.nio.MappedByteBuffer;
-import java.nio.channels.FileChannel;
-import java.util.HashMap;
 import java.util.Locale;
 
 import javax.annotation.Nullable;
@@ -97,9 +90,6 @@ public class LibraryLoader {
     // final (like now) or be protected in some way (volatile of synchronized).
     private final int mLibraryProcessType;
 
-    // Library -> Path it has been loaded from.
-    private final HashMap<String, String> mLoadedFrom;
-
     /**
      * @param libraryProcessType the process the shared library is loaded in. refer to
      *                           LibraryProcessType for possible values.
@@ -119,7 +109,6 @@ public class LibraryLoader {
 
     private LibraryLoader(int libraryProcessType) {
         mLibraryProcessType = libraryProcessType;
-        mLoadedFrom = new HashMap<String, String>();
     }
 
     /**
@@ -204,43 +193,10 @@ public class LibraryLoader {
         }
     }
 
-    private void prefetchLibraryToMemory(Context context, String library) {
-        String libFilePath = mLoadedFrom.get(library);
-        if (libFilePath == null) {
-            Log.i(TAG, "File path not found for " + library);
-            return;
-        }
-        String apkFilePath = context.getApplicationInfo().sourceDir;
-        if (libFilePath.equals(apkFilePath)) {
-            // TODO(lizeb): Make pre-faulting work with libraries loaded from the APK.
-            return;
-        }
-        try {
-            TraceEvent.begin("LibraryLoader.prefetchLibraryToMemory");
-            File file = new File(libFilePath);
-            int size = (int) file.length();
-            FileChannel channel = new RandomAccessFile(file, "r").getChannel();
-            MappedByteBuffer buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, size);
-            // TODO(lizeb): Figure out whether walking the entire library is really necessary.
-            // Page size is 4096 for all current Android architectures.
-            for (int index = 0; index < size; index += 4096) {
-                // Note: Testing shows that neither the Java compiler nor
-                // Dalvik/ART eliminates this loop.
-                buffer.get(index);
-            }
-        } catch (FileNotFoundException e) {
-            Log.w(TAG, "Library file not found: " + e);
-        } catch (IOException e) {
-            Log.w(TAG, "Impossible to map the file: " + e);
-        } finally {
-            TraceEvent.end("LibraryLoader.prefetchLibraryToMemory");
-        }
-    }
-
     /** Prefetches the native libraries in a background thread.
      *
-     * Launches an AsyncTask that maps the native libraries into memory, reads a
-     * part of each page from it, than unmaps it. This is done to warm up the
+     * Launches an AsyncTask that, through a short-lived forked process, reads a
+     * part of each page of the native library.  This is done to warm up the
      * page cache, turning hard page faults into soft ones.
      *
      * This is done this way, as testing shows that fadvise(FADV_WILLNEED) is
@@ -252,15 +208,13 @@ public class LibraryLoader {
         new AsyncTask<Void, Void, Void>() {
             @Override
             protected Void doInBackground(Void... params) {
-                // Note: AsyncTasks are executed in a low priority background
-                // thread, which is the desired behavior here since we don't
-                // want to interfere with the rest of the initialization.
-                for (String library : NativeLibraries.LIBRARIES) {
-                    if (Linker.isChromiumLinkerLibrary(library)) {
-                        continue;
-                    }
-                    prefetchLibraryToMemory(context, library);
+                TraceEvent.begin("LibraryLoader.asyncPrefetchLibrariesToMemory");
+                boolean success = nativeForkAndPrefetchNativeLibrary();
+                if (!success) {
+                    Log.w(TAG, "Forking a process to prefetch the native library failed.");
                 }
+                RecordHistogram.recordBooleanHistogram("LibraryLoader.PrefetchStatus", success);
+                TraceEvent.end("LibraryLoader.asyncPrefetchLibrariesToMemory");
                 return null;
             }
         }.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
@@ -340,7 +294,6 @@ public class LibraryLoader {
                                                 ? "using no map executable support fallback"
                                                 : "directly")
                                         + " from within " + apkFilePath);
-                                mLoadedFrom.put(library, apkFilePath);
                             } else {
                                 // Unpack library fallback.
                                 Log.i(TAG, "Loading " + library
@@ -350,19 +303,10 @@ public class LibraryLoader {
                                         context, library);
                                 fallbackWasUsed = true;
                                 Log.i(TAG, "Built fallback library " + libFilePath);
-                                mLoadedFrom.put(library, libFilePath);
                             }
                         } else {
                             // The library is in its own file.
                             Log.i(TAG, "Loading " + library);
-                            if (context != null) {
-                                ApplicationInfo applicationInfo = context.getApplicationInfo();
-                                File file = new File(applicationInfo.nativeLibraryDir, libFilePath);
-                                mLoadedFrom.put(library, file.getAbsolutePath());
-                            } else {
-                                Log.i(TAG, "No context, cannot locate the native library file for "
-                                        + library);
-                            }
                         }
 
                         // Load the library.
@@ -580,4 +524,9 @@ public class LibraryLoader {
     // Get the version of the native library. This is needed so that we can check we
     // have the right version before initializing the (rest of the) JNI.
     private native String nativeGetVersionNumber();
+
+    // Finds the ranges corresponding to the native library pages, forks a new
+    // process to prefetch these pages and waits for it. The new process then
+    // terminates. This is blocking.
+    private static native boolean nativeForkAndPrefetchNativeLibrary();
 }