[heap] Reland uncommit unused large object page memory.

src/base/platform/platform-linux.cc

 // 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.
 
 // Platform-specific code for Linux goes here. For the POSIX-compatible
 // parts, the implementation is in platform-posix.cc.
 
 #include <pthread.h>
 #include <semaphore.h>
 #include <signal.h>
@@ skipping 357 matching lines @@
 
 bool VirtualMemory::UncommitRegion(void* base, size_t size) {
   return mmap(base,
               size,
               PROT_NONE,
               MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE | MAP_FIXED,
               kMmapFd,
               kMmapFdOffset) != MAP_FAILED;
 }
 
+bool VirtualMemory::ReleasePartialRegion(void* base, size_t size,
+                                         void* free_start, size_t free_size) {
+#if defined(LEAK_SANITIZER)
+  __lsan_unregister_root_region(base, size);
+  __lsan_register_root_region(base, size - free_size);
+#endif
+  return munmap(free_start, free_size) == 0;
+}
 
 bool VirtualMemory::ReleaseRegion(void* base, size_t size) {
 #if defined(LEAK_SANITIZER)
   __lsan_unregister_root_region(base, size);
 #endif
   return munmap(base, size) == 0;
 }
 
 
 bool VirtualMemory::HasLazyCommits() {
   return true;
 }
 
 }  // namespace base
 }  // namespace v8