Check theory that entire mmap page is getting zeroed.

This is a temporary change that will be reverted once crashes can be
analyzed to confirm the working theory.

Bug: 836875
Change-Id: Iaeffbd6c78b604af66b33991392990d71648d4a9
Reviewed-on: https://chromium-review.googlesource.com/1050071
Commit-Queue: Brian White <bcwhite@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557159}

base/metrics/histogram.cc

@@ -32,6 +32,7 @@
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
 #include "base/synchronization/lock.h"
+#include "base/sys_info.h"
 #include "base/values.h"
 #include "build/build_config.h"
 
@@ -548,6 +549,34 @@ void Histogram::ValidateHistogramContents() const {
   CHECK(unlogged_samples_->bucket_ranges());
   CHECK(logged_samples_);
   CHECK(logged_samples_->bucket_ranges());
+#if !defined(OS_NACL)
+  if (0U == logged_samples_->id() && (flags() & kIsPersistent)) {
+    // ID should never be zero. If it is, then it's probably because the
+    // entire memory page was cleared. Check that this is true.
+    // TODO(bcwhite): Remove this.
+    // https://bugs.chromium.org/p/chromium/issues/detail?id=836875
+    const size_t page_size = SysInfo::VMAllocationGranularity();
+    const int* address = reinterpret_cast<const int*>(
+        reinterpret_cast<uintptr_t>(logged_samples_->meta()) &
+        ~(page_size - 1));
+    // Check a couple places so there is evidence in a crash report as to
+    // where it was non-zero.
+    CHECK_EQ(0, address[0]);
+    CHECK_EQ(0, address[1]);
+    CHECK_EQ(0, address[2]);
+    CHECK_EQ(0, address[4]);
+    CHECK_EQ(0, address[8]);
+    CHECK_EQ(0, address[16]);
+    CHECK_EQ(0, address[32]);
+    CHECK_EQ(0, address[64]);
+    CHECK_EQ(0, address[128]);
+    CHECK_EQ(0, address[256]);
+    CHECK_EQ(0, address[512]);
+    // Now check every address.
+    for (size_t i = 0; i < page_size / sizeof(int); ++i)
+      CHECK_EQ(0, address[i]);
+  }
+#endif
   CHECK_NE(0U, logged_samples_->id());
 }
 

base/metrics/histogram_samples.h

@@ -159,6 +159,11 @@ class BASE_EXPORT HistogramSamples {
     return subtle::NoBarrier_Load(&meta_->redundant_count);
   }
 
+  // Temporarily visible for crash debugging. Should be protected.
+  // TODO(bcwhite): Move this back where it belongs.
+  // https://bugs.chromium.org/p/chromium/issues/detail?id=836875
+  Metadata* meta() { return meta_; }
+
  protected:
   enum NegativeSampleReason {
     SAMPLES_HAVE_LOGGED_BUT_NOT_SAMPLE,
@@ -196,8 +201,6 @@ class BASE_EXPORT HistogramSamples {
     return meta_->single_sample;
   }
 
-  Metadata* meta() { return meta_; }
-
  private:
   // Depending on derived class meta values can come from local stoarge or
   // external storage in which case HistogramSamples class cannot take ownership