Provide option to CMake for Page Size (#664)

* Pickup page size from unistd.h

This uses the PAGESIZE constant from the unistd.h on POSIX.
This should make the code more resilient to being compiled on platforms with
different page sizes.

* Allow pagesize to come from cmake.

* Update src/snmalloc/pal/pal_posix.h

Co-authored-by: Nathaniel Filardo <105816689+nwf-msr@users.noreply.github.com>

---------

Co-authored-by: Nathaniel Filardo <105816689+nwf-msr@users.noreply.github.com>

CMakeLists.txt

@@ -65,6 +65,8 @@ endif()
 set(SNMALLOC_MIN_ALLOC_SIZE "" CACHE STRING "Minimum allocation bytes (power of 2)")
 set(SNMALLOC_MIN_ALLOC_STEP_SIZE "" CACHE STRING "Minimum allocation step (power of 2)")
 
+set(SNMALLOC_PAGESIZE "" CACHE STRING "Page size in bytes")
+
 set(SNMALLOC_DEALLOC_BATCH_RING_ASSOC "" CACHE STRING "Associativity of deallocation batch cache; 0 to disable")
 set(SNMALLOC_DEALLOC_BATCH_RING_SET_BITS "" CACHE STRING "Logarithm of number of deallocation batch cache associativity sets")
 
@@ -257,6 +259,8 @@ add_as_define_value(SNMALLOC_MIN_ALLOC_STEP_SIZE)
 add_as_define_value(SNMALLOC_DEALLOC_BATCH_RING_ASSOC)
 add_as_define_value(SNMALLOC_DEALLOC_BATCH_RING_SET_BITS)
 
+add_as_define_value(SNMALLOC_PAGESIZE)
+
 target_compile_definitions(snmalloc INTERFACE $<$<BOOL:CONST_QUALIFIED_MALLOC_USABLE_SIZE>:MALLOC_USABLE_SIZE_QUALIFIER=const>)
 
 # In debug and CI builds, link the backtrace library so that we can get stack

src/snmalloc/pal/pal_posix.h

@@ -8,6 +8,7 @@
 #endif
 #include <errno.h>
 #include <fcntl.h>
+#include <limits.h>
 #include <stdio.h>
 #include <string.h>
 #include <strings.h>
@@ -129,8 +130,15 @@ namespace snmalloc
       | Entropy
 #endif
       ;
-
+#ifdef SNMALLOC_PAGESIZE
+    static_assert(
+      bits::is_pow2(SNMALLOC_PAGESIZE), "Page size must be a power of 2");
+    static constexpr size_t page_size = SNMALLOC_PAGESIZE;
+#elif defined(PAGESIZE)
+    static constexpr size_t page_size = max(Aal::smallest_page_size, PAGESIZE);
+#else
     static constexpr size_t page_size = Aal::smallest_page_size;
+#endif
 
     /**
      * Address bits are potentially mediated by some POSIX OSes, but generally