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

base/android/library_loader/library_prefetcher.cc

Index: base/android/library_loader/library_prefetcher.cc
diff --git a/base/android/library_loader/library_prefetcher.cc b/base/android/library_loader/library_prefetcher.cc
new file mode 100644
index 0000000000000000000000000000000000000000..83ad376e974b7fe4fb14862fd4f80ec2eafa3e62
--- /dev/null
+++ b/base/android/library_loader/library_prefetcher.cc
@@ -0,0 +1,158 @@
+// 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 <sys/resource.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <utility>
+#include <vector>
+
+#include "base/posix/eintr_wrapper.h"
+#include "base/strings/string_util.h"
+
+namespace base {
+namespace android {
+
+namespace {
+
+// Android defines the background priority to this value since at least 2009
+// (see Process.java).
+const int kBackgroundPriority = 10;
+// Valid for all the Android architectures.
+const size_t kPageSize = 4096;
+// We may load directly from the APK.
+const int kSuffixesToMatchCount = 2;
+const char* kSuffixesToMatch[] = {"libchrome.so", "base.apk"};
+
+bool IsReadableAndPrivate(const base::debug::MappedMemoryRegion& region) {
+  return region.permissions & base::debug::MappedMemoryRegion::READ &&
+         region.permissions & base::debug::MappedMemoryRegion::PRIVATE;
+}
+
+bool PathMatchesSuffix(const std::string& path) {
+  for (int i = 0; i < kSuffixesToMatchCount; i++) {

[Robert Sesek, 2015/04/02 15:58:02]

You can remove this in favor of arraysize(kSuffixesToMatch).

[Benoit L, 2015/04/02 16:43:48]

Thank you for the suggestion!
Done.

+    if (EndsWith(path, kSuffixesToMatch[i], true)) {
+      return true;
+    }
+  }
+  return false;
+}
+
+// For each range, reads a byte per page to force it into the page cache.
+// Heap allocations, syscalls and library functions are not allowed in this
+// function.
+// Returns true for success.
+bool Prefetch(const std::vector<std::pair<uintptr_t, uintptr_t>>& ranges) {

[Robert Sesek, 2015/04/02 15:58:02]

Consider making this pair a named type (in the .h private: section):

using PointerRange = std::pair<uintptr_t, uintptr_t>;

[Benoit L, 2015/04/02 16:43:48]

Thank you for the suggestion.
I've updated the other locations to use AddressRange, but not this one. This is
a function included in an anonymous namespace in this file, so I didn't want to
make it a friend of NativeLibraryPrefetcher. But I am totally open to change
this, wdyt?

+  for (const auto& range : ranges) {
+    const uintptr_t page_mask = kPageSize - 1;
+    // If start or end is not page-aligned, parsing went wrong. It is better to
+    // exit with an error.
+    if ((range.first & page_mask) || (range.second & page_mask)) {
+      return false;  // CHECK() is not allowed here.
+    }
+    unsigned char* start_ptr = reinterpret_cast<unsigned char*>(range.first);
+    unsigned char* end_ptr = reinterpret_cast<unsigned char*>(range.second);
+    unsigned char dummy;
+    for (unsigned char* ptr = start_ptr; ptr < end_ptr; ptr += kPageSize) {
+      // Volatile is required to prevent the compiler from eliminating this
+      // loop.
+      dummy ^= *static_cast<volatile unsigned char*>(ptr);
+    }
+  }
+  return true;
+}
+
+}  // namespace
+
+// static
+bool NativeLibraryPrefetcher::IsGoodToPrefetch(
+    const base::debug::MappedMemoryRegion& region) {
+  return PathMatchesSuffix(region.path) &&
+         IsReadableAndPrivate(region);  // .text and .data mappings are private.
+}
+
+// static
+void NativeLibraryPrefetcher::TakeOnlyLibchromeRangesIfPossible(
+    const std::vector<base::debug::MappedMemoryRegion>& regions,
+    std::vector<std::pair<uintptr_t, uintptr_t>>* ranges) {
+  const std::string& libchrome_suffix = kSuffixesToMatch[0];
+  bool has_libchrome_region = false;
+  for (const base::debug::MappedMemoryRegion& region : regions) {
+    if (EndsWith(region.path, libchrome_suffix, true)) {
+      has_libchrome_region = true;
+      break;
+    }
+  }
+  for (const base::debug::MappedMemoryRegion& region : regions) {
+    if (has_libchrome_region &&
+        !EndsWith(region.path, libchrome_suffix, true)) {
+      continue;
+    }
+    ranges->push_back(std::make_pair(region.start, region.end));
+  }
+}
+
+// static
+bool NativeLibraryPrefetcher::FindRanges(
+    std::vector<std::pair<uintptr_t, uintptr_t>>* ranges) {
+  std::string proc_maps;
+  if (!base::debug::ReadProcMaps(&proc_maps))
+    return false;
+  std::vector<base::debug::MappedMemoryRegion> regions;
+  if (!ParseProcMaps(proc_maps, &regions))

[Robert Sesek, 2015/04/02 15:58:02]

base::debug:: ?

[Benoit L, 2015/04/02 16:43:48]

Done.

+    return false;
+
+  std::vector<base::debug::MappedMemoryRegion> regions_to_prefetch;
+  for (const auto& region : regions) {
+    if (IsGoodToPrefetch(region)) {
+      regions_to_prefetch.push_back(region);
+    }
+  }
+
+  TakeOnlyLibchromeRangesIfPossible(regions_to_prefetch, ranges);
+  return true;
+}
+
+// Forks and waits for 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
+//   returns an incorrect address, only the forked process will crash.
+// - Not inflating the memory used by the main process uselessly, which could
+//   increase its likelihood to be killed.
+// The forked process has background priority and, since it is not declared to
+// the android runtime, can be killed at any time, which is not an issue here.
+// static
+bool NativeLibraryPrefetcher::ForkAndPrefetchNativeLibrary() {
+  // Looking for ranges is done before the fork, to avoid syscalls and/or memory
+  // allocations in the forked process. The child process inherits the lock
+  // state of its parent thread. It cannot rely on being able to acquire any
+  // lock (unless special care is taken in a pre-fork handler), including being
+  // able to call malloc().
+  std::vector<std::pair<uintptr_t, uintptr_t>> ranges;
+  if (!FindRanges(&ranges))
+    return false;
+  pid_t pid = fork();
+  if (pid == 0) {
+    setpriority(PRIO_PROCESS, 0, kBackgroundPriority);
+    // _exit() doesn't call the atexit() handlers.
+    _exit(Prefetch(ranges) ? 0 : 1);
+  } else {
+    if (pid < 0) {
+      return false;
+    }
+    int status;
+    const pid_t result = HANDLE_EINTR(waitpid(pid, &status, 0));
+    if (result == pid) {
+      if (WIFEXITED(status)) {
+        return WEXITSTATUS(status) == 0;
+      }
+    }
+    return false;
+  }
+}
+
+}  // namespace android
+}  // namespace base