Modified Metadata range to have separate pool

The Metadata range should not be shared with the object range.  This
change ensures that their are separate requests to the Pal for meta-data
and object data ranges.  The requests are never combined, and thus
memory cannot flow from being used in malloc to later be used in meta-
data.

src/snmalloc/backend/backend.h

@@ -117,24 +117,39 @@ namespace snmalloc
     }
 
     // Global range of memory
-    using StatsR = StatsRange<
+    using GlobalR = GlobalRange<
       LargeBuddyRange<Base, 24, bits::BITS - 1, Pagemap, MinBaseSizeBits()>>;
-    using GlobalR = GlobalRange<StatsR>;
 
 #ifdef SNMALLOC_META_PROTECTED
+    // Introduce two global ranges, so we don't mix Object and Meta
+    using CentralObectRange = GlobalRange<
+      LargeBuddyRange<GlobalR, 24, bits::BITS - 1, Pagemap, MinBaseSizeBits()>>;
+    using CentralMetaRange = GlobalRange<LargeBuddyRange<
+      SubRange<GlobalR, PAL, 6>, // Use SubRange to introduce guard pages.
+      24,
+      bits::BITS - 1,
+      Pagemap,
+      MinBaseSizeBits()>>;
+
     // Source for object allocations
-    using ObjectRange =
-      LargeBuddyRange<CommitRange<GlobalR, PAL>, 21, 21, Pagemap>;
-    // Set up protected range for metadata
-    using SubR = CommitRange<SubRange<GlobalR, PAL, 6>, PAL>;
-    using MetaRange =
-      SmallBuddyRange<LargeBuddyRange<SubR, 21 - 6, bits::BITS - 1, Pagemap>>;
-    using GlobalMetaRange = GlobalRange<MetaRange>;
+    using StatsObject = StatsRange<CommitRange<CentralObectRange, PAL>>;
+    using ObjectRange = LargeBuddyRange<StatsObject, 21, 21, Pagemap>;
+    using StatsR = StatsObject;
+
+    using StatsRMeta = StatsRange<CommitRange<CentralMetaRange, PAL>>;
+
+    using MetaRange = SmallBuddyRange<
+      LargeBuddyRange<StatsRMeta, 21 - 6, bits::BITS - 1, Pagemap>>;
+    // Create global range that can service small meta-data requests.
+    // Don't want to add this to the CentralMetaRange to move Commit outside the
+    // lock on the common case.
+    using GlobalMetaRange = GlobalRange<SmallBuddyRange<StatsRMeta>>;
 #else
     // Source for object allocations and metadata
     // No separation between the two
+    using StatsR = StatsRange<GlobalR>;
     using ObjectRange = SmallBuddyRange<
-      LargeBuddyRange<CommitRange<GlobalR, PAL>, 21, 21, Pagemap>>;
+      LargeBuddyRange<CommitRange<StatsR, PAL>, 21, 21, Pagemap>>;
     using GlobalMetaRange = GlobalRange<ObjectRange>;
 #endif
 
@@ -307,13 +322,23 @@ namespace snmalloc
     static size_t get_current_usage()
     {
       StatsR stats_state;
-      return stats_state.get_current_usage();
+      auto result = stats_state.get_current_usage();
+#ifdef SNMALLOC_PROTECT_METADATA
+      StatsRMeta stats_state_meta;
+      result += stats_state_meta.get_current_usage();
+#endif
+      return result;
     }
 
     static size_t get_peak_usage()
     {
       StatsR stats_state;
-      return stats_state.get_peak_usage();
+      auto result = stats_state.get_peak_usage();
+#ifdef SNMALLOC_PROTECT_METADATA
+      StatsRMeta stats_state_meta;
+      result += stats_state_meta.get_peak_usage();
+#endif
+      return result;
     }
   };
 } // namespace snmalloc