vm: implement a sparse memory model

vm/flat.c

@@ -1,3 +1,20 @@
+/* ### The flat memory model ###
+
+   under this memory model, the system memory is represented by
+   a single contiguous array of vm_pages. this array spans from
+   physical address up to the last available byte, as provided by
+   memblock. any extra reserved regions after the last available
+   byte will not be included to save memory.
+
+   this memory model is good for systems with a smaller amount of
+   physical memory that is mostly contiguous with few holes or
+   reserved regions. it is simpler and has less overhead.
+
+   for systems with a large amount of memory, or with large
+   amounts of reserved memory (especially those whose reserved
+   memory outstripts free memory), the sparse memory model may
+   be a better choice.
+ */
 #include <socks/vm.h>
 #include <socks/memblock.h>
 #include <socks/printk.h>
@@ -8,7 +25,7 @@ static vm_page_t *page_array = NULL;
 /* number of pages stored in page_array */
 static size_t page_array_count = 0;
 
-void vm_flat_init()
+void vm_flat_init(void)
 {
 	printk("vm: using flat memory model");
 	size_t pmem_size = 0;