Move BACKEND_MARKER to backend/metatypes

And rename it to REMOTE_BACKEND_MARKER to scope it a bit.

src/backend/largebuddyrange.h

@@ -29,7 +29,8 @@ namespace snmalloc
     {
       SNMALLOC_ASSERT((r & (MIN_CHUNK_SIZE - 1)) == 0);
       // Preserve lower bits.
-      *ptr = r | address_cast(*ptr & (MIN_CHUNK_SIZE - 1)) | BACKEND_MARKER;
+      *ptr = r | address_cast(*ptr & (MIN_CHUNK_SIZE - 1)) |
+        MetaEntry::REMOTE_BACKEND_MARKER;
     }
 
     static Contents get(const Holder* ptr)

src/backend/metatypes.h

@@ -123,6 +123,17 @@ namespace snmalloc
     uintptr_t remote_and_sizeclass{0};
 
   public:
+    /**
+     * This bit is set in remote_and_sizeclass to discriminate between the case
+     * that it is in use by the frontend (0) or by the backend (1).  For the
+     * former case, see mem/metaslab.h; for the latter, see backend/backend.h
+     * and backend/largebuddyrange.h.
+     *
+     * This value is statically checked by the frontend to ensure that its
+     * bit packing does not conflict; see mem/remoteallocator.h
+     */
+    static constexpr address_t REMOTE_BACKEND_MARKER = 1 << 7;
+
     constexpr MetaEntry() = default;
 
     /**

src/mem/remoteallocator.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include "../backend/metatypes.h"
 #include "../mem/allocconfig.h"
 #include "../mem/freelist.h"
 #include "../mem/sizeclasstable.h"
@@ -15,14 +16,12 @@ namespace snmalloc
   static constexpr size_t REMOTE_MIN_ALIGN =
     bits::max<size_t>(CACHELINE_SIZE, SIZECLASS_REP_SIZE) << 1;
 
-  // This bit is set on the RemoteAllocator* to indicate it is
-  // actually being used by the backend for some other use.
-  static constexpr size_t BACKEND_MARKER = REMOTE_MIN_ALIGN >> 1;
-
   // The bit above the sizeclass is always zero unless this is used
   // by the backend to represent another datastructure such as the buddy
   // allocator entries.
-  constexpr size_t REMOTE_WITH_BACKEND_MARKER_ALIGN = BACKEND_MARKER;
+  constexpr size_t REMOTE_WITH_BACKEND_MARKER_ALIGN =
+    MetaEntry::REMOTE_BACKEND_MARKER;
+  static_assert((REMOTE_MIN_ALIGN >> 1) == MetaEntry::REMOTE_BACKEND_MARKER);
 
   /**
    * Global key for all remote lists.

src/mem/remotecache.h

@@ -108,7 +108,7 @@ namespace snmalloc
             // set implies this is used by the backend, and we should not be
             // deallocating memory here.
             snmalloc_check_client(
-              (address_cast(remote) & BACKEND_MARKER) == 0,
+              (address_cast(remote) & MetaEntry::REMOTE_BACKEND_MARKER) == 0,
               "Delayed detection of attempt to free internal structure.");
             if constexpr (SharedStateHandle::Options.QueueHeadsAreTame)
             {