The last freelist entry should not be discarded when using OS_WIN.

DiscardVirtualMemory makes the contents of discarded memory undefined.
c.f. https://msdn.microsoft.com/ja-jp/library/windows/desktop/dn781432(v=vs.85).aspx

BUG=

Change-Id: Ifd06e6f75271dc6a38cf0a34027cb20f1a887cfe
Reviewed-on: https://chromium-review.googlesource.com/571621
Commit-Queue: Takashi Sakamoto <tasak@google.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486700}

base/allocator/partition_allocator/partition_alloc.cc

@@ -1100,7 +1100,11 @@ static size_t PartitionPurgePage(PartitionPage* page, bool discard) {
   DCHECK(page->num_unprovisioned_slots < bucket_num_slots);
   size_t num_slots = bucket_num_slots - page->num_unprovisioned_slots;
   char slot_usage[max_slot_count];
+#if !defined(OS_WIN)
+  // The last freelist entry should not be discarded when using OS_WIN.
+  // DiscardVirtualMemory makes the contents of discarded memory undefined.
   size_t last_slot = static_cast<size_t>(-1);
+#endif
   memset(slot_usage, 1, num_slots);
   char* ptr = reinterpret_cast<char*>(PartitionPageToPointer(page));
   PartitionFreelistEntry* entry = page->freelist_head;
@@ -1111,6 +1115,7 @@ static size_t PartitionPurgePage(PartitionPage* page, bool discard) {
     DCHECK(slotIndex < num_slots);
     slot_usage[slotIndex] = 0;
     entry = PartitionFreelistMask(entry->next);
+#if !defined(OS_WIN)
     // If we have a slot where the masked freelist entry is 0, we can
     // actually discard that freelist entry because touching a discarded
     // page is guaranteed to return original content or 0.
@@ -1118,6 +1123,7 @@ static size_t PartitionPurgePage(PartitionPage* page, bool discard) {
     // because the masking function is negation.)
     if (!PartitionFreelistMask(entry))
       last_slot = slotIndex;
+#endif
   }
 
   // If the slot(s) at the end of the slot span are not in used, we can
@@ -1162,7 +1168,9 @@ static size_t PartitionPurgePage(PartitionPage* page, bool discard) {
       *entry_ptr = PartitionFreelistMask(entry);
       entry_ptr = reinterpret_cast<PartitionFreelistEntry**>(entry);
       num_new_entries++;
+#if !defined(OS_WIN)
       last_slot = slotIndex;
+#endif
     }
     // Terminate the freelist chain.
     *entry_ptr = nullptr;
@@ -1185,8 +1193,12 @@ static size_t PartitionPurgePage(PartitionPage* page, bool discard) {
     // null, we can discard that pointer value too.
     char* begin_ptr = ptr + (i * slot_size);
     char* end_ptr = begin_ptr + slot_size;
+#if !defined(OS_WIN)
     if (i != last_slot)
       begin_ptr += sizeof(PartitionFreelistEntry);
+#else
+    begin_ptr += sizeof(PartitionFreelistEntry);
+#endif
     begin_ptr = reinterpret_cast<char*>(
         RoundUpToSystemPage(reinterpret_cast<size_t>(begin_ptr)));
     end_ptr = reinterpret_cast<char*>(

base/allocator/partition_allocator/partition_alloc_unittest.cc

@@ -1839,14 +1839,22 @@ TEST_F(PartitionAllocTest, PurgeDiscardable) {
       EXPECT_TRUE(stats);
       EXPECT_TRUE(stats->is_valid);
       EXPECT_EQ(0u, stats->decommittable_bytes);
+#if defined(OS_WIN)
+      EXPECT_EQ(0u, stats->discardable_bytes);
+#else
       EXPECT_EQ(kSystemPageSize, stats->discardable_bytes);
+#endif
       EXPECT_EQ(kSystemPageSize, stats->active_bytes);
       EXPECT_EQ(2 * kSystemPageSize, stats->resident_bytes);
     }
     CheckPageInCore(ptr1 - kPointerOffset, true);
     PartitionPurgeMemoryGeneric(generic_allocator.root(),
                                 PartitionPurgeDiscardUnusedSystemPages);
+#if defined(OS_WIN)
+    CheckPageInCore(ptr1 - kPointerOffset, true);
+#else
     CheckPageInCore(ptr1 - kPointerOffset, false);
+#endif
 
     PartitionFreeGeneric(generic_allocator.root(), ptr2);
   }
@@ -1969,7 +1977,11 @@ TEST_F(PartitionAllocTest, PurgeDiscardable) {
       EXPECT_TRUE(stats);
       EXPECT_TRUE(stats->is_valid);
       EXPECT_EQ(0u, stats->decommittable_bytes);
+#if defined(OS_WIN)
+      EXPECT_EQ(kSystemPageSize, stats->discardable_bytes);
+#else
       EXPECT_EQ(2 * kSystemPageSize, stats->discardable_bytes);
+#endif
       EXPECT_EQ(kSystemPageSize, stats->active_bytes);
       EXPECT_EQ(4 * kSystemPageSize, stats->resident_bytes);
     }
@@ -1981,7 +1993,11 @@ TEST_F(PartitionAllocTest, PurgeDiscardable) {
                                 PartitionPurgeDiscardUnusedSystemPages);
     EXPECT_EQ(1u, page->num_unprovisioned_slots);
     CheckPageInCore(ptr1 - kPointerOffset, true);
+#if defined(OS_WIN)
+    CheckPageInCore(ptr1 - kPointerOffset + kSystemPageSize, true);
+#else
     CheckPageInCore(ptr1 - kPointerOffset + kSystemPageSize, false);
+#endif
     CheckPageInCore(ptr1 - kPointerOffset + (kSystemPageSize * 2), true);
     CheckPageInCore(ptr1 - kPointerOffset + (kSystemPageSize * 3), false);