Re-enable size class checks when freeing spans

Performance overhead of these checks was determined to be negligible.

PiperOrigin-RevId: 816792943
Change-Id: I223cb605b0c774f9c317685b5b053aa9b3e09a38

Author: toddlipcon

tcmalloc/central_freelist.cc

@@ -74,16 +74,18 @@ Length StaticForwarder::class_to_pages(int size_class) {
 
 void StaticForwarder::MapObjectsToSpans(absl::Span<void*> batch, Span** spans,
                                         int expected_size_class) {
-  // TODO(b/396002858): Resume using expected_size_class or remove it.
-  (void)expected_size_class;
-
   // Prefetch Span objects to reduce cache misses.
   for (int i = 0; i < batch.size(); ++i) {
     const PageId p = PageIdContaining(batch[i]);
-    Span* span = tc_globals.pagemap().GetExistingDescriptor(p);
+    auto [span, page_size_class] =
+        tc_globals.pagemap().GetExistingDescriptorAndSizeClass(p);
     if (ABSL_PREDICT_FALSE(span == nullptr)) {
       ReportDoubleFree(tc_globals, batch[i]);
     }
+    if (ABSL_PREDICT_FALSE(page_size_class != expected_size_class)) {
+      ReportMismatchedSizeClass(tc_globals, span, page_size_class,
+                                expected_size_class);
+    }
     span->Prefetch();
     spans[i] = span;
   }

tcmalloc/testing/memory_errors_test.cc

@@ -699,9 +699,7 @@ TEST_F(TcMallocTest, MismatchedDeleteTooSmall) {
   }
 }
 
-TEST_F(TcMallocTest, DISABLED_MismatchedSizeClassInFreelistInsertion) {
-  // TODO(b/396002858): Reenable this test.
-
+TEST_F(TcMallocTest, MismatchedSizeClassInFreelistInsertion) {
 #ifdef ABSL_HAVE_ADDRESS_SANITIZER
   GTEST_SKIP() << "ASan will trap ahead of us";
 #endif