[heap] Ensure unmapper task doesn't start during GC

There is now only one invocation left of
MemoryAllocator::Unmapper::FreeQueuedChunks in the GC epilogue.

Bug: chromium:1329064, chromium:1327132
Change-Id: Icc21ada4c5a8a9505ed6435ef1f62fe48b2dbb52
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3667079
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80745}

src/heap/heap.cc

@@ -1458,6 +1458,14 @@ void Heap::GarbageCollectionEpilogueInSafepoint(GarbageCollector collector) {
     ReduceNewSpaceSize();
   }
 
+  // Ensure that unmapper task isn't running during full GC. We need access to
+  // those pages for accessing page flags when processing old-to-new slots.
+  DCHECK_IMPLIES(collector == GarbageCollector::MARK_COMPACTOR,
+                 !memory_allocator()->unmapper()->IsRunning());
+
+  // Start concurrent unmapper tasks to free pages queued during GC.
+  memory_allocator()->unmapper()->FreeQueuedChunks();
+
   // Remove CollectionRequested flag from main thread state, as the collection
   // was just performed.
   safepoint()->AssertActive();

src/heap/large-spaces.cc

@@ -522,14 +522,12 @@ void NewLargeObjectSpace::FreeDeadObjects(
     const std::function<bool(HeapObject)>& is_dead) {
   bool is_marking = heap()->incremental_marking()->IsMarking();
   size_t surviving_object_size = 0;
-  bool freed_pages = false;
   PtrComprCageBase cage_base(heap()->isolate());
   for (auto it = begin(); it != end();) {
     LargePage* page = *it;
     it++;
     HeapObject object = page->GetObject();
     if (is_dead(object)) {
-      freed_pages = true;
       RemovePage(page);
       heap()->memory_allocator()->Free(MemoryAllocator::FreeMode::kConcurrently,
                                        page);
@@ -543,9 +541,6 @@ void NewLargeObjectSpace::FreeDeadObjects(
   // Right-trimming does not update the objects_size_ counter. We are lazily
   // updating it after every GC.
   objects_size_ = surviving_object_size;
-  if (freed_pages) {
-    heap()->memory_allocator()->unmapper()->FreeQueuedChunks();
-  }
 }
 
 void NewLargeObjectSpace::SetCapacity(size_t capacity) {

src/heap/mark-compact.cc

@@ -1107,11 +1107,10 @@ void MarkCompactCollector::Finish() {
 
   sweeper()->StartSweeperTasks();
 
-  // Give pages that are queued to be freed back to the OS. Ensure unmapper
-  // tasks are stopped such that queued pages aren't freed before this point. We
-  // still need all pages to be accessible for the "update pointers" phase.
+  // Ensure unmapper tasks are stopped such that queued pages aren't freed
+  // before this point. We still need all pages to be accessible for the "update
+  // pointers" phase.
   DCHECK(!heap_->memory_allocator()->unmapper()->IsRunning());
-  heap()->memory_allocator()->unmapper()->FreeQueuedChunks();
 
   // Shrink pages if possible after processing and filtering slots.
   ShrinkPagesToObjectSizes(heap(), heap()->lo_space());
@@ -5777,8 +5776,6 @@ void MinorMarkCompactCollector::EvacuatePrologue() {
 void MinorMarkCompactCollector::EvacuateEpilogue() {
   SemiSpaceNewSpace::From(heap()->new_space())
       ->set_age_mark(heap()->new_space()->top());
-  // Give pages that are queued to be freed back to the OS.
-  heap()->memory_allocator()->unmapper()->FreeQueuedChunks();
 }
 
 int MinorMarkCompactCollector::CollectToSpaceUpdatingItems(

src/heap/memory-allocator.cc

@@ -104,6 +104,8 @@ class MemoryAllocator::Unmapper::UnmapFreeMemoryJob : public JobTask {
 };
 
 void MemoryAllocator::Unmapper::FreeQueuedChunks() {
+  if (NumberOfChunks() == 0) return;
+
   if (!heap_->IsTearingDown() && FLAG_concurrent_sweeping) {
     if (job_handle_ && job_handle_->IsValid()) {
       job_handle_->NotifyConcurrencyIncrease();

src/heap/new-spaces.cc

@@ -161,7 +161,6 @@ void SemiSpace::Uncommit() {
   DCHECK_EQ(CommittedMemory(), removed_page_size);
   DCHECK_EQ(CommittedPhysicalMemory(), 0);
   AccountUncommitted(removed_page_size);
-  heap()->memory_allocator()->unmapper()->FreeQueuedChunks();
   DCHECK(!IsCommitted());
 }
 
@@ -225,7 +224,6 @@ void SemiSpace::ShrinkTo(size_t new_capacity) {
     int delta_pages = static_cast<int>(delta / Page::kPageSize);
     RewindPages(delta_pages);
     AccountUncommitted(delta);
-    heap()->memory_allocator()->unmapper()->FreeQueuedChunks();
   }
   target_capacity_ = new_capacity;
 }