Prefetch the native library from native code by parsing /proc/pid/maps.

base/android/library_loader/library_prefetcher.cc

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "base/android/library_loader/library_prefetcher.h"
+
+#include <stdlib.h>
+#include <sys/resource.h>
+#include <unistd.h>
+#include <utility>
+#include <vector>
+
+namespace base {
+namespace android {
+
+FileLineIterator::FileLineIterator(const base::FilePath& path) {
+  file_ = base::OpenFile(path, "r");
+}
+
+FileLineIterator::~FileLineIterator() {
+  if (file_) {
+    base::CloseFile(file_);
+  }
+}
+
+std::string FileLineIterator::NextLine() {
+  if (!file_) {
+    return std::string();
+  }
+  char* line = fgets(line_buffer_, kMaxLineLength - 1, file_);
+  return line ? std::string(line) : std::string();
+}
+
+ProcMapsIterator::ProcMapsIterator(FileLineIterator* line_iterator)
+    : line_iterator_(line_iterator) {
+}
+
+bool ProcMapsIterator::Next(uint64_t* start,
+                            uint64_t* end,
+                            std::string* flags,
+                            uint64_t* offset,
+                            std::string* device,
+                            int64_t* inode,
+                            std::string* filename) {
+  std::string line = line_iterator_->NextLine();
+  if (line.size() == 0) {
+    return false;
+  }
+  // Example:
+  // b6e62000-b6e64000 r-xp 00000000 b3:15 1204 /system/lib/libstdc++.so
+  // start-end flags offset device inode [filename]
+  std::string line_str(line);
+  std::vector<std::string> pieces;
+  base::SplitStringAlongWhitespace(line_str, &pieces);
+  size_t pieces_count = pieces.size();
+  // Filename is optional
+  DCHECK((pieces_count == 5) || (pieces_count == 6));
+
+  const char* start_end = pieces[0].c_str();
+  char* delimiter_ptr, *dummy;
+  static_assert(sizeof(long long) == sizeof(int64_t),
+                "long long is not int64_t");  // This way we can use strtoll().
+  *start = strtoull(start_end, &delimiter_ptr, 16);
+  *end = strtoull(delimiter_ptr + 1, &dummy, 16);
+  *flags = pieces[1];
+  *offset = strtoull(pieces[2].c_str(), &dummy, 16);
+  *device = pieces[3];
+  *inode = strtoll(pieces[4].c_str(), &dummy, 10);
+  if (pieces_count == 5) {
+    *filename = std::string();
+  } else {
+    *filename = pieces[5];
+  }
+  return true;
+}
+
+namespace {
+
+// Android defines the background priority to this value since at least 2009
+// (see Process.java).
+const int kBackgroundPriority = 10;
+// We may load directly from the APK.
+const int kSuffixesToMatchCount = 2;
+const char* kSuffixesToMatch[] = {"libchrome.so", "base.apk"};
+
+bool IsReadableAndPrivate(const char* flags) {
+  return flags && flags[0] == 'r' && flags[3] == 'p';
+}
+
+bool FilenameMatchesSuffix(const char* filename) {
+  if (!filename)
+    return false;
+  const size_t filename_length = strlen(filename);
+  for (int i = 0; i < kSuffixesToMatchCount; i++) {
+    const size_t suffix_length = strlen(kSuffixesToMatch[i]);
+    if (filename_length < suffix_length)
+      continue;
+    if (!strcmp(kSuffixesToMatch[i],
+                filename + filename_length - suffix_length))
+      return true;
+  }
+  return false;
+}
+
+// For each range, reads a byte per page to force it into the page cache.
+void Prefetch(const std::vector<std::pair<uint64_t, uint64_t>>& ranges) {
+  for (const auto& range : ranges) {
+    unsigned char* start_address = (unsigned char*)range.first;

[petrcermak, 2015/03/23 15:00:48]

nit: There should be a space after the closing parenthesis before "range"
(applies to the line below as well)

[Benoit L, 2015/03/23 16:41:58]

Done.

+    unsigned char* end_address = (unsigned char*)range.second;
+    unsigned char dummy;
+    for (unsigned char* addr = start_address; addr < end_address - sizeof(int);
+         addr += 4096) {

[petrcermak, 2015/03/23 15:00:47]

Shouldn't this be a constant (e.g. kPageSize)?

[Benoit L, 2015/03/23 16:41:58]

Done.

+      // Volatile is required to prevent the compiler from eliminating this
+      // loop.
+      dummy ^= *((volatile unsigned char*) addr);
+    }
+  }
+}
+
+}  // namespace
+
+std::vector<std::pair<uint64_t, uint64_t>> FindRanges(
+    FileLineIterator* file_line_iterator) {
+  std::vector<std::pair<uint64_t, uint64_t>> ranges;
+  ProcMapsIterator maps_iterator(file_line_iterator);
+  uint64_t start, end, offset;
+  int64_t inode;
+  std::string flags, device, filename;
+
+  while (maps_iterator.Next(&start, &end, &flags, &offset, &device, &inode,
+                            &filename)) {
+    if (FilenameMatchesSuffix(filename.c_str()) &&
+        IsReadableAndPrivate(flags.c_str())) {
+      ranges.push_back(std::make_pair(start, end));
+    }
+  }
+  return ranges;
+}
+
+// Forks a process prefetching the native library. This is done in a forked
+// process for the following reasons:
+// - Isolating the main process from mistakes in the parsing. If the parsing

[petrcermak, 2015/03/23 15:00:47]

This doesn't seem to be the case here. If I understand your code correctly, all
the parsing is done inside FindRanges(), which is done *before* the fork.

[Benoit L, 2015/03/23 16:41:58]

I was talking about the fact that we might get incorrect addresses. Since
actually accessing the addresses is done in a separate process, the parsing can
be wrong without crashing the app.

+//  returns an incorrect address, only the forked process will crash.

[petrcermak, 2015/03/23 15:00:47]

nit: Incorrect indentation of this line.

[Benoit L, 2015/03/23 16:41:58]

Done.

+// - Not inflating the memory used by the main process uselessly, which could
+//   increase its likelihood to be killed.
+// - Have the pages clean and not referenced, to make them easy for the kernel
+//   to evict, minimizing disruption caused by this prefetching.
+// The forked process has background priority, and since it is not declared to

[petrcermak, 2015/03/23 15:00:47]

nit: The first comma should actually appear *after* the "and" here (to enclose
"since ... runtime" in commas).

[Benoit L, 2015/03/23 16:41:58]

Done.

+// the android runtime, can be killed at any time, which is not an issue here.
+void ForkAndPrefetchNativeLibrary() {
+  // Looking for ranges is done before the fork, to avoid syscalls and/or
+  // memory allocations in the forked process, that can be problematic.
+  FileLineIterator file_line_iterator(base::FilePath("/proc/self/maps"));
+  std::vector<std::pair<uint64_t, uint64_t>> ranges =
+      FindRanges(&file_line_iterator);
+  pid_t pid = fork();
+  if (pid == 0) {
+    setpriority(PRIO_PROCESS, 0, kBackgroundPriority);
+    Prefetch(ranges);
+    exit(0);
+  }
+}
+
+}  // namespace android
+}  // namespace base