Re-land "Concurrently unmap free pages."

src/heap/spaces.h

Index: src/heap/spaces.h
diff --git a/src/heap/spaces.h b/src/heap/spaces.h
index c00a84fb9b8a55c5da92ddd823b98918aea3097a..e9a208502f89c3881ced3d9ad8fb216f0ad1f7df 100644
--- a/src/heap/spaces.h
+++ b/src/heap/spaces.h
@@ -980,10 +980,15 @@ class CodeRange {
     size_t size;
   };
 
+  // All access to free_list_ require to take the free_list_mutex_. GC threads
+  // may access the free_list_ concurrently to the main thread.
+  base::Mutex free_list_mutex_;
+
   // Freed blocks of memory are added to the free list.  When the allocation
   // list is exhausted, the free list is sorted and merged to make the new
   // allocation list.
   List<FreeBlock> free_list_;
+
   // Memory is allocated from the free blocks on the allocation list.
   // The block at current_allocation_block_index_ is the current block.
   List<FreeBlock> allocation_list_;
@@ -1079,6 +1084,15 @@ class MemoryAllocator {
   LargePage* AllocateLargePage(intptr_t object_size, Space* owner,
                                Executability executable);
 
+  // PreFree logically frees the object, i.e., it takes care of the size
+  // bookkeeping and calls the allocation callback.

[Michael Lippautz, 2015/08/24 13:43:12]

Can we extend this comment to clarify that this code is meant to be run on the
main thread?

+  void PreFreeMemory(MemoryChunk* chunk);
+
+  // FreeMemory can be called concurrently when PreFree was executed before.
+  void PerformFreeMemory(MemoryChunk* chunk);
+
+  // Free is a wrapper method, which calls PreFree and PerformFreeMemory
+  // together.
   void Free(MemoryChunk* chunk);
 
   // Returns the maximum available bytes of heaps.
@@ -1128,6 +1142,8 @@ class MemoryAllocator {
 
   bool CommitMemory(Address addr, size_t size, Executability executable);
 
+  void FreeNewSpaceMemory(Address addr, base::VirtualMemory* reservation,
+                          Executability executable);
   void FreeMemory(base::VirtualMemory* reservation, Executability executable);
   void FreeMemory(Address addr, size_t size, Executability executable);