sandbox: vm: add page splitting, merging, and allocation

sandbox/vm/zone.c

@@ -1,3 +1,4 @@
+#include "socks/queue.h"
 #include <socks/types.h>
 #include <socks/vm.h>
 #include <string.h>
@@ -126,3 +127,80 @@ void vm_zone_init(vm_zone_t *z, const vm_zone_descriptor_t *zone_info)
 		convert_region_to_blocks(z, block_start, block_end + VM_PAGE_SIZE - 1, this_page_reserved);
 	}
 }
+
+static int replenish_free_page_list(vm_zone_t *z, vm_page_order_t order)
+{
+	if (queue_length(&z->z_free_pages[order]) != 0) {
+		/* we already have pages available. */
+		return 0;
+	}
+	
+	if (order == VM_PAGE_MAX_ORDER) {
+		/* there are no larger pages to split, so just give up. */
+		return -1;
+	}
+	
+	/* the lowest page order that is >= `order` and still has pages available */
+	vm_page_order_t first_order_with_free = VM_MAX_PAGE_ORDERS;
+
+	for (vm_page_order_t i = order; i <= VM_PAGE_MAX_ORDER; i++) {
+		if (queue_length(&z->z_free_pages[i]) > 0) {
+			first_order_with_free = i;
+			break;
+		}
+	}
+
+	if (first_order_with_free == VM_MAX_PAGE_ORDERS) {
+		/* there are no pages available to split */
+		return -1;
+	}
+
+	if (first_order_with_free == order) {
+		/* there are free pages of the requested order, so nothing needs to be done */
+		return 0;
+	}
+
+	/* starting from the first page list with free pages,
+	   take a page, split it in half, and add the sub-pages
+	   to the next order's free list. */
+	for (vm_page_order_t i = first_order_with_free; i > order; i--) {
+		queue_entry_t *pg_entry = queue_pop_front(&z->z_free_pages[order]);
+		vm_page_t *pg = QUEUE_CONTAINER(vm_page_t, p_free_list, pg_entry);
+
+		vm_page_t *a, *b;
+		vm_page_split(pg, &a, &b);
+
+		queue_push_back(&z->z_free_pages[order - 1], &a->p_free_list);
+		queue_push_back(&z->z_free_pages[order - 1], &b->p_free_list);
+	}
+
+	/* handle the last order separately. if the requested order is 0 (4K)
+	   handling it within the for-loop would cause an underflow */
+	queue_entry_t *pg_entry = queue_pop_front(&z->z_free_pages[order + 1]);
+	vm_page_t *pg = QUEUE_CONTAINER(vm_page_t, p_free_list, pg_entry);
+
+	vm_page_t *a, *b;
+	vm_page_split(pg, &a, &b);
+
+	queue_push_back(&z->z_free_pages[order - 1], &a->p_free_list);
+	queue_push_back(&z->z_free_pages[order - 1], &b->p_free_list);
+		
+	return 0;
+}
+
+vm_page_t *vm_zone_alloc_page(vm_zone_t *z, vm_page_order_t order, vm_flags_t flags)
+{
+	int result = replenish_free_page_list(z, order);
+	if (result != 0) {
+		return NULL;
+	}
+
+	
+	queue_entry_t *pg_entry = queue_pop_front(&z->z_free_pages[order]);
+	return QUEUE_CONTAINER(vm_page_t, p_free_list, pg_entry);
+}
+
+void vm_zone_free_page(vm_zone_t *z, vm_page_t *pg)
+{
+
+}