[heap] Uncommit unused large object page memory.

src/base/platform/platform.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This module contains the platform-specific code. This make the rest of the
 // code less dependent on operating system, compilers and runtime libraries.
 // This module does specifically not deal with differences between different
 // processor architecture.
 // The platform classes have the same definition for all platforms. The
 // implementation for a particular platform is put in platform_<os>.cc.
@@ skipping 319 matching lines @@
 
   // Commits real memory. Returns whether the operation succeeded.
   bool Commit(void* address, size_t size, bool is_executable);
 
   // Uncommit real memory.  Returns whether the operation succeeded.
   bool Uncommit(void* address, size_t size);
 
   // 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
     // inside the allocated region.
     void* address = address_;
     size_t size = size_;
     CHECK(InVM(address, size));
     Reset();
     bool result = ReleaseRegion(address, size);
     USE(result);
@@ skipping 12 matching lines @@
   static void* ReserveRegion(size_t size);
 
   static bool CommitRegion(void* base, size_t size, bool is_executable);
 
   static bool UncommitRegion(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.
   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.
   static bool HasLazyCommits();
 
  private:
   bool InVM(void* address, size_t size) {
     return (reinterpret_cast<uintptr_t>(address_) <=
             reinterpret_cast<uintptr_t>(address)) &&
            ((reinterpret_cast<uintptr_t>(address_) + size_) >=
@@ skipping 107 matching lines @@
   int stack_size_;
   Semaphore* start_semaphore_;
 
   DISALLOW_COPY_AND_ASSIGN(Thread);
 };
 
 }  // namespace base
 }  // namespace v8
 
 #endif  // V8_BASE_PLATFORM_PLATFORM_H_