Fix accounting for reused memory for v8 on macOS.

Calls to madvise(MADV_FREE_REUSABLE) [when discarding/decommitting memory]
should be paired with calls to madvise(MADV_FREE_REUSE) [when reusing/committing
memory]. The latter is purely for accounting purposes.

Bug: chromium:823915
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Ib1758fd72c5ad4dfe731f5d9a6dbaf75b1e0e14b
Reviewed-on: https://chromium-review.googlesource.com/988193
Commit-Queue: Erik Chen <erikchen@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52674}

src/base/platform/platform-posix.cc

@@ -354,6 +354,18 @@ bool OS::SetPermissions(void* address, size_t size, MemoryPermission access) {
   if (ret == 0 && access == OS::MemoryPermission::kNoAccess) {
     ret = ReclaimInaccessibleMemory(address, size);
   }
+
+// For accounting purposes, we want to call MADV_FREE_REUSE on macOS after
+// changing permissions away from OS::MemoryPermission::kNoAccess. Since this
+// state is not kept at this layer, we always call this if access != kNoAccess.
+// The cost is a syscall that effectively no-ops.
+// TODO(erikchen): Fix this to only call MADV_FREE_REUSE when necessary.
+// https://crbug.com/823915
+#if defined(OS_MACOSX)
+  if (access != OS::MemoryPermission::kNoAccess)
+    madvise(address, size, MADV_FREE_REUSE);
+#endif
+
   return ret == 0;
 }