[heap] Uncommit unused large object page memory.

src/base/platform/platform.h

Index: src/base/platform/platform.h
diff --git a/src/base/platform/platform.h b/src/base/platform/platform.h
index 9464fb11231dbcd59efed2846ff4d4894681f83f..73300d937942ee7170babc659f0371cfb7308646 100644
--- a/src/base/platform/platform.h
+++ b/src/base/platform/platform.h
@@ -337,6 +337,21 @@ class VirtualMemory {
   // Creates a single guard page at the given address.
   bool Guard(void* address);
 
+  // Releases the tail [free_start, free_start + size] of a previously allocated
+  // chunk of memory.
+  void ReleasePartial(void* free_start, size_t size) {

[ulan, 2016/06/06 13:44:22]

As discussed, we can make a stricter version of this function that releases the
tail of the region.

[Hannes Payer (out of office), 2016/06/06 14:42:44]

Done.

+    DCHECK(IsReserved());
+    // Notice: Order is important here. The VirtualMemory object might live
+    // inside the allocated region.
+    CHECK(InVM(free_start, size));
+    DCHECK(free_start != address_);
+    DCHECK(size < size_);
+    bool result = ReleasePartialRegion(address_, size_, free_start, size);
+    USE(result);
+    DCHECK(result);
+    size_ -= size;
+  }
+
   void Release() {
     DCHECK(IsReserved());
     // Notice: Order is important here. The VirtualMemory object might live
@@ -369,6 +384,12 @@ class VirtualMemory {
   // and the same size it was reserved with.
   static bool ReleaseRegion(void* base, size_t size);
 
+  // Must be called with a base pointer that has been returned by ReserveRegion
+  // and the same size it was reserved with.
+  // [free_start, free_start + free_size] is the memory that will be released.
+  static bool ReleasePartialRegion(void* base, size_t size, void* free_start,
+                                   size_t free_size);
+
   // Returns true if OS performs lazy commits, i.e. the memory allocation call
   // defers actual physical memory allocation till the first memory access.
   // Otherwise returns false.