Remove at_least

The Pal was providing policy for overallocating a block of memory to
achieve alignment make that part of the backend.
The backend should be responsible for layout policy.

src/backend/address_space.h

@@ -96,12 +96,21 @@ namespace snmalloc
           else if constexpr (!pal_supports<NoAllocation, PAL>)
           {
             // Need at least 2 times the space to guarantee alignment.
-            // Hold lock here as a race could cause additional requests to
-            // the PAL, and this could lead to suprious OOM.  This is
-            // particularly bad if the PAL gives all the memory on first call.
-            auto block_and_size = PAL::reserve_at_least(size * 2);
-            block = CapPtr<void, CBChunk>(block_and_size.first);
-            block_size = block_and_size.second;
+            size_t needed_size = size * 2;
+            // Magic number (27) for over-allocating a block of memory
+            // These should be further refined based on experiments.
+            constexpr size_t min_size = bits::one_at_bit(27);
+            for (size_t size_request = bits::max(needed_size, min_size);
+                 size_request >= needed_size;
+                 size_request = size_request / 2)
+            {
+              block = CapPtr<void, CBChunk>(PAL::reserve(size_request));
+              if (block != nullptr)
+              {
+                block_size = size_request;
+                break;
+              }
+            }
 
             // Ensure block is pointer aligned.
             if (

src/backend/backend.h

@@ -11,7 +11,7 @@ namespace snmalloc
    * This class implements the standard backend for handling allocations.
    * It abstracts page table management and address space management.
    */
-  template<typename PAL, bool fixed_range>
+  template<SNMALLOC_CONCEPT(ConceptPAL) PAL, bool fixed_range>
   class BackendAllocator
   {
   public:

src/backend/pagemap.h

@@ -119,9 +119,7 @@ namespace snmalloc
 
       // TODO request additional space, and move to random offset.
 
-      // TODO wasting space if size2 bigger than needed.
-      auto [new_body_untyped, size2] =
-        Pal::reserve_at_least(ENTRIES * sizeof(T));
+      auto new_body_untyped = Pal::reserve(ENTRIES * sizeof(T));
 
       auto new_body = reinterpret_cast<T*>(new_body_untyped);