[offthread] Ensure off-thread page marking bits are right

Ensure that the off-thread pages' marking bits (including the page
headers) are correct, and synchronised correctly on merge.

Bug: chromium:1011762
Change-Id: I46c66fb35d49d39eb0da3513c869baf49c366706
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2110020
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66805}

src/heap/heap.cc

@@ -6666,7 +6666,7 @@ bool Heap::PageFlagsAreConsistent(HeapObject object) {
   CHECK_EQ(chunk->InReadOnlySpace(), slim_chunk->InReadOnlySpace());
 
   // Marking consistency.
-  if (chunk->IsWritable()) {
+  if (chunk->IsWritable() && !Heap::InOffThreadSpace(object)) {
     // RO_SPACE can be shared between heaps, so we can't use RO_SPACE objects to
     // find a heap. The exception is when the ReadOnlySpace is writeable, during
     // bootstrapping, so explicitly allow this case.

src/heap/off-thread-factory.cc

@@ -169,8 +169,7 @@ void OffThreadFactory::Publish(Isolate* isolate) {
             isolate->factory()->InternalizeString(string_handle);
 
         // Recalculate the slot in case there was GC and the holder moved.
-        ObjectSlot slot(heap_object_handles[i]->ptr() +
-                        string_slots_[i].slot_offset);
+        ObjectSlot slot(heap_object_handles[i]->ptr() + slot_offset);
 
         DCHECK(string_handle->IsThinString() ||
                string_handle->IsInternalizedString());

src/heap/spaces.cc

@@ -762,7 +762,8 @@ Page* PagedSpace::InitializePage(MemoryChunk* chunk) {
       page->area_size());
   // Make sure that categories are initialized before freeing the area.
   page->ResetAllocationStatistics();
-  page->SetOldGenerationPageFlags(heap()->incremental_marking()->IsMarking());
+  page->SetOldGenerationPageFlags(!is_off_thread_space() &&
+                                  heap()->incremental_marking()->IsMarking());
   page->AllocateFreeListCategories();
   page->InitializeFreeListCategories();
   page->list_node().Initialize();
@@ -1744,6 +1745,7 @@ void PagedSpace::MergeLocalSpace(LocalSpace* other) {
 
     if (merging_from_off_thread) {
       DCHECK_NULL(p->sweeping_slot_set());
+      p->SetOldGenerationPageFlags(heap()->incremental_marking()->IsMarking());
       if (heap()->incremental_marking()->black_allocation()) {
         p->CreateBlackArea(p->area_start(), p->HighWaterMark());
       }
@@ -1912,7 +1914,7 @@ int PagedSpace::CountTotalPages() {
 
 void PagedSpace::SetLinearAllocationArea(Address top, Address limit) {
   SetTopAndLimit(top, limit);
-  if (top != kNullAddress && top != limit &&
+  if (top != kNullAddress && top != limit && !is_off_thread_space() &&
       heap()->incremental_marking()->black_allocation()) {
     Page::FromAllocationAreaAddress(top)->CreateBlackArea(top, limit);
   }