cppgc: young-gen: Avoid eager commits of the age-table

The CL fixes PMF regressions that happend after increasing
the AgeTable size.

Bug: chromium:1336529
Change-Id: If1f099b43bfcb3a8c7dd4a1c229fcb08735eb744
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3707098Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81180}

include/cppgc/internal/caged-heap-local-data.h

@@ -61,7 +61,7 @@ class V8_EXPORT AgeTable final {
   Age GetAgeForRange(uintptr_t cage_offset_begin,
                      uintptr_t cage_offset_end) const;
 
-  void Reset(PageAllocator* allocator);
+  void ResetForTesting();
 
  private:
   V8_INLINE size_t card(uintptr_t offset) const {
@@ -98,10 +98,6 @@ struct CagedHeapLocalData final {
 #if defined(CPPGC_YOUNG_GENERATION)
   AgeTable age_table;
 #endif
-
- private:
-  friend class CagedHeap;
-  explicit CagedHeapLocalData(PageAllocator&);
 };
 
 }  // namespace internal

src/heap/cppgc/caged-heap-local-data.cc

@@ -13,12 +13,6 @@
 namespace cppgc {
 namespace internal {
 
-CagedHeapLocalData::CagedHeapLocalData(PageAllocator& allocator) {
-#if defined(CPPGC_YOUNG_GENERATION)
-  age_table.Reset(&allocator);
-#endif  // defined(CPPGC_YOUNG_GENERATION)
-}
-
 #if defined(CPPGC_YOUNG_GENERATION)
 
 static_assert(
@@ -63,17 +57,8 @@ AgeTable::Age AgeTable::GetAgeForRange(uintptr_t offset_begin,
   return result;
 }
 
-void AgeTable::Reset(PageAllocator* allocator) {
-  // TODO(chromium:1029379): Consider MADV_DONTNEED instead of MADV_FREE on
-  // POSIX platforms.
+void AgeTable::ResetForTesting() {
   std::fill(table_.begin(), table_.end(), Age::kOld);
-  const uintptr_t begin = RoundUp(reinterpret_cast<uintptr_t>(table_.data()),
-                                  allocator->CommitPageSize());
-  const uintptr_t end =
-      RoundDown(reinterpret_cast<uintptr_t>(table_.data() + table_.size()),
-                allocator->CommitPageSize());
-
-  allocator->DiscardSystemPages(reinterpret_cast<void*>(begin), end - begin);
 }
 
 #endif  // defined(CPPGC_YOUNG_GENERATION)

src/heap/cppgc/caged-heap.cc

@@ -118,8 +118,6 @@ CagedHeap::CagedHeap(PageAllocator& platform_allocator)
   // Failing to commit the reservation means that we are out of memory.
   CHECK(is_not_oom);
 
-  new (cage_start) CagedHeapLocalData(platform_allocator);
-
   const CagedAddress caged_heap_start = RoundUp(
       reinterpret_cast<CagedAddress>(cage_start) + sizeof(CagedHeapLocalData),
       kPageSize);

test/unittests/heap/cppgc/age-table-unittest.cc

@@ -22,7 +22,7 @@ class AgeTableTest : public testing::TestWithHeap {
         age_table_(CagedHeapLocalData::Get().age_table) {}
 
   ~AgeTableTest() override {
-    age_table_.Reset(GetPlatform().GetPageAllocator());
+    age_table_.ResetForTesting();
     // Collect all allocated pages.
     for (auto* page : allocated_pages_) BasePage::Destroy(page);
   }