[Memory] Rename OS::ReleasePartialRegion to OS::Release.

- Change VirtualMemory to match OS memory concepts. Rename Release
  Free, ReleasePartial to Release.
- Adds comments to make the semantics clear. Right now V8 munmaps
  on POSIX, making address space available, while on Windows it is
  only possible to decommit.

Bug: chromium:756050
Change-Id: I6ba04d857ab9e1ca1f273e9e766e0825e67210cc
Reviewed-on: https://chromium-review.googlesource.com/783513
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49586}

src/allocation.cc

@@ -129,7 +129,7 @@ VirtualMemory::VirtualMemory(size_t size, void* hint, size_t alignment)
 
 VirtualMemory::~VirtualMemory() {
   if (IsReserved()) {
-    Release();
+    Free();
   }
 }
 
@@ -147,8 +147,10 @@ bool VirtualMemory::SetPermissions(void* address, size_t size,
   return result;
 }
 
-size_t VirtualMemory::ReleasePartial(void* free_start) {
+size_t VirtualMemory::Release(void* free_start) {
   DCHECK(IsReserved());
+  DCHECK(IsAddressAligned(static_cast<Address>(free_start),
+                          base::OS::CommitPageSize()));
   // Notice: Order is important here. The VirtualMemory object might live
   // inside the allocated region.
   const size_t free_size = size_ - (reinterpret_cast<size_t>(free_start) -
@@ -161,14 +163,12 @@ size_t VirtualMemory::ReleasePartial(void* free_start) {
   __lsan_unregister_root_region(address_, size_);
   __lsan_register_root_region(address_, size_ - free_size);
 #endif
-  const bool result = base::OS::ReleasePartialRegion(free_start, free_size);
-  USE(result);
-  DCHECK(result);
+  CHECK(base::OS::Release(free_start, free_size));
   size_ -= free_size;
   return free_size;
 }
 
-void VirtualMemory::Release() {
+void VirtualMemory::Free() {
   DCHECK(IsReserved());
   // Notice: Order is important here. The VirtualMemory object might live
   // inside the allocated region.

src/allocation.h

@@ -133,10 +133,11 @@ class V8_EXPORT_PRIVATE VirtualMemory {
   bool SetPermissions(void* address, size_t size,
                       base::OS::MemoryPermission access);
 
-  // Releases the memory after |free_start|. Returns the bytes released.
-  size_t ReleasePartial(void* free_start);
+  // Releases memory after |free_start|. Returns the number of bytes released.
+  size_t Release(void* free_start);
 
-  void Release();
+  // Frees all memory.
+  void Free();
 
   // Assign control of the reserved region to a different VirtualMemory object.
   // The old object is no longer functional (IsReserved() returns false).

src/base/platform/platform-cygwin.cc

@@ -141,6 +141,13 @@ bool OS::Free(void* address, const size_t size) {
   return VirtualFree(address, 0, MEM_RELEASE) != 0;
 }
 
+// static
+bool OS::Release(void* address, size_t size) {
+  DCHECK_EQ(0, reinterpret_cast<uintptr_t>(address) % CommitPageSize());
+  DCHECK_EQ(0, size % CommitPageSize());
+  return VirtualFree(address, size, MEM_DECOMMIT) != 0;
+}
+
 // static
 bool OS::SetPermissions(void* address, size_t size, MemoryPermission access) {
   DCHECK_EQ(0, reinterpret_cast<uintptr_t>(address) % CommitPageSize());
@@ -152,11 +159,6 @@ bool OS::SetPermissions(void* address, size_t size, MemoryPermission access) {
   return VirtualAlloc(address, size, MEM_COMMIT, protect) != nullptr;
 }
 
-// static
-bool OS::ReleasePartialRegion(void* address, size_t size) {
-  return VirtualFree(address, size, MEM_DECOMMIT) != 0;
-}
-
 // static
 bool OS::HasLazyCommits() {
   // TODO(alph): implement for the platform.

src/base/platform/platform-fuchsia.cc

@@ -99,6 +99,14 @@ bool OS::Free(void* address, const size_t size) {
                        reinterpret_cast<uintptr_t>(address), size) == ZX_OK;
 }
 
+// static
+bool OS::Release(void* address, size_t size) {
+  DCHECK_EQ(0, reinterpret_cast<uintptr_t>(address) % CommitPageSize());
+  DCHECK_EQ(0, size % CommitPageSize());
+  return zx_vmar_unmap(zx_vmar_root_self(),
+                       reinterpret_cast<uintptr_t>(address), size) == ZX_OK;
+}
+
 // static
 bool OS::SetPermissions(void* address, size_t size, MemoryPermission access) {
   DCHECK_EQ(0, reinterpret_cast<uintptr_t>(address) % CommitPageSize());
@@ -109,12 +117,6 @@ bool OS::SetPermissions(void* address, size_t size, MemoryPermission access) {
                          prot) == ZX_OK;
 }
 
-// static
-bool OS::ReleasePartialRegion(void* address, size_t size) {
-  return zx_vmar_unmap(zx_vmar_root_self(),
-                       reinterpret_cast<uintptr_t>(address), size) == ZX_OK;
-}
-
 // static
 bool OS::HasLazyCommits() {
   // TODO(scottmg): Port, https://crbug.com/731217.

src/base/platform/platform-posix.cc

@@ -275,6 +275,13 @@ bool OS::Free(void* address, const size_t size) {
   return munmap(address, size) == 0;
 }
 
+// static
+bool OS::Release(void* address, size_t size) {
+  DCHECK_EQ(0, reinterpret_cast<uintptr_t>(address) % CommitPageSize());
+  DCHECK_EQ(0, size % CommitPageSize());
+  return munmap(address, size) == 0;
+}
+
 // static
 bool OS::SetPermissions(void* address, size_t size, MemoryPermission access) {
   DCHECK_EQ(0, reinterpret_cast<uintptr_t>(address) % CommitPageSize());
@@ -283,11 +290,6 @@ bool OS::SetPermissions(void* address, size_t size, MemoryPermission access) {
   return mprotect(address, size, prot) == 0;
 }
 
-// static
-bool OS::ReleasePartialRegion(void* address, size_t size) {
-  return munmap(address, size) == 0;
-}
-
 // static
 bool OS::HasLazyCommits() {
 #if V8_OS_AIX || V8_OS_LINUX || V8_OS_MACOSX

src/base/platform/platform-win32.cc

@@ -823,6 +823,13 @@ bool OS::Free(void* address, const size_t size) {
   return VirtualFree(address, 0, MEM_RELEASE) != 0;
 }
 
+// static
+bool OS::Release(void* address, size_t size) {
+  DCHECK_EQ(0, reinterpret_cast<uintptr_t>(address) % CommitPageSize());
+  DCHECK_EQ(0, size % CommitPageSize());
+  return VirtualFree(address, size, MEM_DECOMMIT) != 0;
+}
+
 // static
 bool OS::SetPermissions(void* address, size_t size, MemoryPermission access) {
   DCHECK_EQ(0, reinterpret_cast<uintptr_t>(address) % CommitPageSize());
@@ -834,11 +841,6 @@ bool OS::SetPermissions(void* address, size_t size, MemoryPermission access) {
   return VirtualAlloc(address, size, MEM_COMMIT, protect) != nullptr;
 }
 
-// static
-bool OS::ReleasePartialRegion(void* address, size_t size) {
-  return VirtualFree(address, size, MEM_DECOMMIT) != 0;
-}
-
 // static
 bool OS::HasLazyCommits() {
   // TODO(alph): implement for the platform.

src/base/platform/platform.h

@@ -187,15 +187,17 @@ class V8_BASE_EXPORT OS {
   // multiples of AllocatePageSize(). Returns true on success, otherwise false.
   V8_WARN_UNUSED_RESULT static bool Free(void* address, const size_t size);
 
+  // Releases memory that is no longer needed. The range specified by address
+  // and size must be part of an allocated memory region, and must be multiples
+  // of CommitPageSize(). Released memory is left in an undefined state, so it
+  // should not be accessed. Returns true on success, otherwise false.
+  V8_WARN_UNUSED_RESULT static bool Release(void* address, size_t size);
+
   // Sets permissions according to the access argument. address and size must be
   // multiples of CommitPageSize(). Returns true on success, otherwise false.
   V8_WARN_UNUSED_RESULT static bool SetPermissions(void* address, size_t size,
                                                    MemoryPermission access);
 
-  // Release part of a reserved address range.
-  V8_WARN_UNUSED_RESULT static bool ReleasePartialRegion(void* address,
-                                                         size_t size);
-
   static bool HasLazyCommits();
 
   // Sleep for a specified time interval.

src/heap/spaces.cc

@@ -305,7 +305,7 @@ void MemoryAllocator::TearDown() {
   capacity_ = 0;
 
   if (last_chunk_.IsReserved()) {
-    last_chunk_.Release();
+    last_chunk_.Free();
   }
 
   delete code_range_;
@@ -434,7 +434,7 @@ void MemoryAllocator::FreeMemory(VirtualMemory* reservation,
   DCHECK(executable == NOT_EXECUTABLE || !code_range()->valid() ||
          reservation->size() <= Page::kPageSize);
 
-  reservation->Release();
+  reservation->Free();
 }
 
 
@@ -488,9 +488,9 @@ Address MemoryAllocator::AllocateAlignedMemory(
   }
 
   if (base == nullptr) {
-    // Failed to commit the body. Release the mapping and any partially
-    // committed regions inside it.
-    reservation.Release();
+    // Failed to commit the body. Free the mapping and any partially committed
+    // regions inside it.
+    reservation.Free();
     size_.Decrement(reserve_size);
     return nullptr;
   }
@@ -981,7 +981,7 @@ void MemoryAllocator::PartialFreeMemory(MemoryChunk* chunk, Address start_free,
   // 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);
+  const size_t released_bytes = reservation->Release(start_free);
   DCHECK_GE(size_.Value(), released_bytes);
   size_.Decrement(released_bytes);
   isolate_->counters()->memory_allocated()->Decrement(

src/heap/spaces.h

@@ -1032,7 +1032,7 @@ class CodeRange {
  public:
   explicit CodeRange(Isolate* isolate);
   ~CodeRange() {
-    if (virtual_memory_.IsReserved()) virtual_memory_.Release();
+    if (virtual_memory_.IsReserved()) virtual_memory_.Free();
   }
 
   // Reserves a range of virtual memory, but does not commit any of it.

src/heap/store-buffer.cc

@@ -68,7 +68,7 @@ void StoreBuffer::SetUp() {
 
 
 void StoreBuffer::TearDown() {
-  if (virtual_memory_.IsReserved()) virtual_memory_.Release();
+  if (virtual_memory_.IsReserved()) virtual_memory_.Free();
   top_ = nullptr;
   for (int i = 0; i < kStoreBuffers; i++) {
     start_[i] = nullptr;

src/wasm/wasm-heap.cc

@@ -872,7 +872,7 @@ void WasmCodeManager::Free(VirtualMemory* mem) {
   void* start = mem->address();
   void* end = mem->end();
   size_t size = mem->size();
-  mem->Release();
+  mem->Free();
   TRACE_HEAP("VMem Release: %p:%p (%zu)\n", start, end, size);
 }