diff --git a/runtime/gc/allocator/rosalloc.cc b/runtime/gc/allocator/rosalloc.cc
index 8b125dd..2c487fe 100644
--- a/runtime/gc/allocator/rosalloc.cc
+++ b/runtime/gc/allocator/rosalloc.cc
@@ -58,10 +58,16 @@
       page_release_mode_(page_release_mode),
       page_release_size_threshold_(page_release_size_threshold),
       is_running_on_memory_tool_(running_on_memory_tool) {
+  DCHECK_ALIGNED(base, kPageSize);
   DCHECK_EQ(RoundUp(capacity, kPageSize), capacity);
   DCHECK_EQ(RoundUp(max_capacity, kPageSize), max_capacity);
   CHECK_LE(capacity, max_capacity);
   CHECK_ALIGNED(page_release_size_threshold_, kPageSize);
+  // Zero the memory explicitly (don't rely on that the mem map is zero-initialized).
+  if (!kMadviseZeroes) {
+    memset(base_, 0, max_capacity);
+  }
+  CHECK_EQ(madvise(base_, max_capacity, MADV_DONTNEED), 0);
   if (!initialized_) {
     Initialize();
   }
diff --git a/runtime/gc/allocator/rosalloc.h b/runtime/gc/allocator/rosalloc.h
index a472a8b..b12cb5b 100644
--- a/runtime/gc/allocator/rosalloc.h
+++ b/runtime/gc/allocator/rosalloc.h
@@ -192,6 +192,7 @@
         Verify();
       }
       DCHECK(slot != nullptr);
+      DCHECK(slot->Next() == nullptr);
       Slot** headp = reinterpret_cast<Slot**>(&head_);
       Slot** tailp = kUseTail ? reinterpret_cast<Slot**>(&tail_) : nullptr;
       Slot* old_head = *headp;
diff --git a/runtime/gc/space/memory_tool_malloc_space-inl.h b/runtime/gc/space/memory_tool_malloc_space-inl.h
index ea8b8aa..6cb2465 100644
--- a/runtime/gc/space/memory_tool_malloc_space-inl.h
+++ b/runtime/gc/space/memory_tool_malloc_space-inl.h
@@ -240,9 +240,9 @@
                     kAdjustForRedzoneInAllocSize,
                     kUseObjSizeForUsable>::MemoryToolMallocSpace(
     MemMap* mem_map, size_t initial_size, Params... params) : S(mem_map, initial_size, params...) {
-  MEMORY_TOOL_MAKE_DEFINED(mem_map->Begin(), initial_size);
-  MEMORY_TOOL_MAKE_UNDEFINED(mem_map->Begin() + initial_size,
-                     mem_map->Size() - initial_size);
+  // Don't want to change the valgrind states of the mem map here as the allocator is already
+  // initialized at this point and that may interfere with what the allocator does internally. Note
+  // that the tail beyond the initial size is mprotected.
 }
 
 template <typename S,