[heap] Fix memory allocator counters for partially releasing pages

Bug: chromium:724947
Change-Id: I287677b2cf18154bcbc0d0a5b15d12455d73d0c3
Reviewed-on: https://chromium-review.googlesource.com/534153Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45923}

src/base/platform/platform.h

@@ -342,21 +342,22 @@ class V8_BASE_EXPORT VirtualMemory {
   // Creates a single guard page at the given address.
   bool Guard(void* address);
 
-  // Releases the memory after |free_start|.
-  void ReleasePartial(void* free_start) {
+  // Releases the memory after |free_start|. Returns the bytes released.
+  size_t ReleasePartial(void* free_start) {
     DCHECK(IsReserved());
     // Notice: Order is important here. The VirtualMemory object might live
     // inside the allocated region.
-    size_t size = size_ - (reinterpret_cast<size_t>(free_start) -
-                           reinterpret_cast<size_t>(address_));
+    const size_t size = size_ - (reinterpret_cast<size_t>(free_start) -
+                                 reinterpret_cast<size_t>(address_));
     CHECK(InVM(free_start, size));
     DCHECK_LT(address_, free_start);
     DCHECK_LT(free_start, reinterpret_cast<void*>(
                               reinterpret_cast<size_t>(address_) + size_));
-    bool result = ReleasePartialRegion(address_, size_, free_start, size);
+    const bool result = ReleasePartialRegion(address_, size_, free_start, size);
     USE(result);
     DCHECK(result);
     size_ -= size;
+    return size;
   }
 
   void Release() {

src/heap/spaces.cc

@@ -874,10 +874,6 @@ void MemoryAllocator::PartialFreeMemory(MemoryChunk* chunk, Address start_free,
                                         size_t bytes_to_free) {
   base::VirtualMemory* reservation = chunk->reserved_memory();
   DCHECK(reservation->IsReserved());
-  DCHECK_GE(size_.Value(), bytes_to_free);
-  size_.Decrement(bytes_to_free);
-  isolate_->counters()->memory_allocated()->Decrement(
-      static_cast<int>(bytes_to_free));
   chunk->size_ -= bytes_to_free;
   chunk->area_end_ -= bytes_to_free;
   if (chunk->IsFlagSet(MemoryChunk::IS_EXECUTABLE)) {
@@ -885,9 +881,16 @@ void MemoryAllocator::PartialFreeMemory(MemoryChunk* chunk, Address start_free,
                      static_cast<uintptr_t>(GetCommitPageSize()));
     DCHECK_EQ(chunk->address() + chunk->size(),
               chunk->area_end() + CodePageGuardSize());
-    chunk->reservation_.Guard(chunk->area_end_);
-  }
-  reservation->ReleasePartial(start_free);
+    reservation->Guard(chunk->area_end_);
+  }
+  // On e.g. Windows, a reservation may be larger than a page and releasing
+  // partially starting at |start_free| will also release the potentially
+  // unused part behind the current page.
+  const size_t released_bytes = reservation->ReleasePartial(start_free);
+  DCHECK_GE(size_.Value(), released_bytes);
+  size_.Decrement(released_bytes);
+  isolate_->counters()->memory_allocated()->Decrement(
+      static_cast<int>(released_bytes));
 }
 
 void MemoryAllocator::PreFreeMemory(MemoryChunk* chunk) {