NFC: Feed Pagemap its primitive allocator as template arg

This will let us use Pagemaps further down the dependency stack (specifically,
we're going to want a Pagemap inside the AddressSpaceManager) by letting us
manually tie the knot rather than rely on GlobalVirtual and
default_memory_provider() being defined by the time we want a Pagemap.

src/mem/chunkmap.h

@@ -81,7 +81,7 @@ namespace snmalloc
   using ChunkmapPagemap = std::conditional_t<
     CHUNKMAP_USE_FLATPAGEMAP,
     FlatPagemap<SUPERSLAB_BITS, uint8_t>,
-    Pagemap<SUPERSLAB_BITS, uint8_t, 0>>;
+    Pagemap<SUPERSLAB_BITS, uint8_t, 0, DefaultPrimAlloc>>;
 
   struct ForChunkmap
   {};

src/mem/largealloc.h

@@ -390,4 +390,13 @@ namespace snmalloc
   {
     return *(Singleton<GlobalVirtual*, GlobalVirtual::make>::get());
   }
+
+  struct DefaultPrimAlloc
+  {
+    template<typename T, size_t alignment, typename... Args>
+    static T* alloc_chunk(Args&&... args)
+    {
+      return default_memory_provider().alloc_chunk<T, alignment>(args...);
+    }
+  };
 } // namespace snmalloc

src/mem/pagemap.h

@@ -53,8 +53,24 @@ namespace snmalloc
    * allocators do not interact with this directly but rather via the
    * static ChunkMap object, which encapsulates knowledge about the
    * pagemap's parametric type T.
+   *
+   * The other template paramters are...
+   *
+   *   GRANULARITY_BITS: the log2 of the size in bytes of the address space
+   *   granule associated with each entry.
+   *
+   *   default_content: An initial value of T (typically "0" or something akin)
+   *
+   *   PrimAlloc: A class used to source PageMap-internal memory; it must have a
+   *   method callable as if it had the following type:
+   *
+   *      template<typename T, size_t alignment> static T* alloc_chunk(void);
    */
-  template<size_t GRANULARITY_BITS, typename T, T default_content>
+  template<
+    size_t GRANULARITY_BITS,
+    typename T,
+    T default_content,
+    typename PrimAlloc>
   class Pagemap
   {
   private:
@@ -154,8 +170,7 @@ namespace snmalloc
         if (e->compare_exchange_strong(
               value, LOCKED_ENTRY, std::memory_order_relaxed))
         {
-          auto& v = default_memory_provider();
-          value = v.alloc_chunk<PagemapEntry, OS_PAGE_SIZE>();
+          value = PrimAlloc::template alloc_chunk<PagemapEntry, OS_PAGE_SIZE>();
           e->store(value, std::memory_order_release);
         }
         else

src/test/func/pagemap/pagemap.cc

@@ -15,7 +15,7 @@ using namespace snmalloc;
 using T = size_t;
 static constexpr size_t GRANULARITY_BITS = 9;
 static constexpr T PRIME = 251;
-Pagemap<GRANULARITY_BITS, T, 0> pagemap_test;
+Pagemap<GRANULARITY_BITS, T, 0, DefaultPrimAlloc> pagemap_test;
 
 int main(int argc, char** argv)
 {