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 <stdio.h>
+#include <sys/resource.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <utility>
+#include <vector>
+
+#include "base/logging.h"
+#include "base/posix/eintr_wrapper.h"
+#include "base/strings/string_number_conversions.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 std::string& flags) {
+  return flags.size() == 4 && flags[0] == 'r' && flags[3] == 'p';
+}
+
+bool FilenameMatchesSuffix(const std::string& filename) {
+  for (int i = 0; i < kSuffixesToMatchCount; i++) {
+    if (EndsWith(filename, 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.
+void Prefetch(const std::vector<std::pair<uint64_t, uint64_t>>& ranges) {
+  for (const auto& range : ranges) {
+    const uint64_t page_mask = kPageSize - 1;
+    CHECK(!(range.first & page_mask) && !(range.second & page_mask));
+    unsigned char* start_address = (unsigned char*)range.first;

[pasko, 2015/03/31 12:01:43]

C-style casts are discouraged, please use the least aggressive suitable C++ cast
;)

[Benoit L, 2015/03/31 14:00:01]

Done.

+    unsigned char* end_address = (unsigned char*)range.second;
+    unsigned char dummy;
+    for (unsigned char* addr = start_address; addr < end_address;
+         addr += kPageSize) {
+      // Volatile is required to prevent the compiler from eliminating this
+      // loop.
+      dummy ^= *((volatile unsigned char*)addr);
+    }
+  }
+}
+
+}  // namespace
+
+// static
+bool NativeLibraryPrefetcher::ParseMapping(const std::string& line,
+                                           Mapping* mapping) {
+  // Example:
+  // b6e62000-b6e64000 r-xp 00000000 b3:15 1204 /system/lib/libstdc++.so
+  // start-end flags offset device inode [filename]
+  std::vector<std::string> pieces;
+  base::SplitStringAlongWhitespace(line, &pieces);
+  size_t pieces_count = pieces.size();
+  // Filename is optional, so at least 5 fields.
+  if (pieces_count < 5) {
+    return false;
+  }
+
+  std::vector<std::string> start_end;
+  base::SplitString(pieces[0], '-', &start_end);
+  if (start_end.size() != 2) {
+    return false;
+  }
+  bool ok = base::HexStringToUInt64(start_end[0], &mapping->start);
+  ok &= base::HexStringToUInt64(start_end[1], &mapping->end);
+  mapping->flags = pieces[1];
+  // Discards fields 3, 4, and 5 starting from 1 (ie offset, device and inode).

[pasko, 2015/03/31 12:01:43]

We use this form 'Discards' and 'Truncates' when we describe a function or a
class or a file. In the implementation itself it is more common to say
'Discard', 'Truncate'. Please do so.

[Benoit L, 2015/03/31 14:00:01]

Done.

+  if (!ok) {
+    return false;
+  }
+  if (pieces_count == 5) {
+    mapping->filename = std::string();
+  } else {
+    // Truncates filenames containing spaces, but these are not the droids
+    // you're looking for.
+    mapping->filename = pieces[5];
+  }
+  return true;
+}
+
+// static
+bool NativeLibraryPrefetcher::IsGoodToPrefetch(const Mapping& mapping) {
+  return FilenameMatchesSuffix(mapping.filename) &&
+      IsReadableAndPrivate(mapping.flags);  // Code mappings are private.

[pasko, 2015/03/31 12:01:43]

nit:

s/Code/Code and data/

?

[Benoit L, 2015/03/31 14:00:01]

Done.

+}
+
+// static
+bool NativeLibraryPrefetcher::FindRanges(
+    std::vector<std::pair<uint64_t, uint64_t>>* ranges) {
+  base::ScopedFILE file(base::OpenFile(base::FilePath("/proc/self/maps"), "r"));
+  if (!file.get()) {
+    return false;
+  }
+  std::vector<Mapping> mappings;
+  const size_t kMaxLineLength = 4096;
+  char line_buffer[kMaxLineLength];
+  while (char* line_str = fgets(line_buffer, kMaxLineLength - 1, file.get())) {
+    std::string line(line_str);
+    Mapping mapping;
+    if (!ParseMapping(line, &mapping)) {
+      continue;
+    }
+    if (IsGoodToPrefetch(mapping)) {
+      mappings.push_back(mapping);
+    }
+  }
+
+  // If libchrome.so is mapped, keep only libchrome's ranges.

[pasko, 2015/03/31 12:01:43]

please move it to another function and test separately

TakeOnlyLibchromeRangesIfPossible() or something ..

[Benoit L, 2015/03/31 14:00:01]

Done.

+  const std::string& libchrome_suffix = kSuffixesToMatch[0];
+  bool has_libchrome_mapping = false;
+  for (const Mapping& mapping : mappings) {
+    if (EndsWith(mapping.filename, libchrome_suffix, true)) {
+      has_libchrome_mapping = true;
+      break;
+    }
+  }
+  for (const Mapping& mapping : mappings) {
+    if (has_libchrome_mapping
+        && !EndsWith(mapping.filename, libchrome_suffix, true)) {
+      continue;
+    }
+    ranges->push_back(std::make_pair(mapping.start, mapping.end));
+  }
+  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 all the parent thread locks. 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<uint64_t, uint64_t>> ranges;
+  if (!FindRanges(&ranges)) {
+    return false;
+  }
+  pid_t pid = fork();
+  if (pid == 0) {
+    setpriority(PRIO_PROCESS, 0, kBackgroundPriority);
+    Prefetch(ranges);
+    _exit(0);  // Don't call the atexit() handlers.
+  } 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