diff --git a/gc.c b/gc.c
index a1ed084d23..dcdc8fd4c8 100644
--- a/gc.c
+++ b/gc.c
@@ -3213,20 +3213,6 @@ Init_heap(void)
 {
     rb_objspace_t *objspace = &rb_objspace;
 
-#if defined(HAVE_MMAP) && !HAVE_CONST_PAGE_SIZE && !defined(PAGE_MAX_SIZE)
-    /* Need to determine if we can use mmap at runtime. */
-# ifdef PAGE_SIZE
-    /* If the PAGE_SIZE macro can be used. */
-    use_mmap_aligned_alloc = PAGE_SIZE <= HEAP_PAGE_SIZE;
-# elif defined(HAVE_SYSCONF) && defined(_SC_PAGE_SIZE)
-    /* If we can use sysconf to determine the page size. */
-    use_mmap_aligned_alloc = sysconf(_SC_PAGE_SIZE) <= HEAP_PAGE_SIZE;
-# else
-    /* Otherwise we can't determine the system page size, so don't use mmap. */
-    use_mmap_aligned_alloc = FALSE;
-# endif
-#endif
-
 #if defined(HAVE_SYSCONF) && defined(_SC_PAGE_SIZE)
     /* If Ruby's heap pages are not a multiple of the system page size, we
      * cannot use mprotect for the read barrier, so we must disable automatic
@@ -3238,6 +3224,20 @@ Init_heap(void)
     }
 #endif
 
+#if defined(HAVE_MMAP) && !HAVE_CONST_PAGE_SIZE && !defined(PAGE_MAX_SIZE)
+    /* Need to determine if we can use mmap at runtime. */
+# ifdef PAGE_SIZE
+    /* If the PAGE_SIZE macro can be used. */
+    use_mmap_aligned_alloc = PAGE_SIZE <= HEAP_PAGE_SIZE;
+# elif defined(HAVE_SYSCONF) && defined(_SC_PAGE_SIZE)
+    /* If we can use sysconf to determine the page size. */
+    use_mmap_aligned_alloc = pagesize <= HEAP_PAGE_SIZE;
+# else
+    /* Otherwise we can't determine the system page size, so don't use mmap. */
+    use_mmap_aligned_alloc = FALSE;
+# endif
+#endif
+
     objspace->next_object_id = INT2FIX(OBJ_ID_INITIAL);
     objspace->id_to_obj_tbl = st_init_table(&object_id_hash_type);
     objspace->obj_to_id_tbl = st_init_numtable();