vm: object: implement creating copy-on-write duplicates of vm-objects

include/kernel/vm-object.h

@@ -84,6 +84,9 @@ extern struct vm_object *vm_object_create_in_place(
 	size_t data_len,
 	vm_prot_t prot);
 
+/* create a copy-on-write duplicate of a vm-object */
+extern struct vm_object *vm_object_duplicate_cow(struct vm_object *vmo);
+
 extern struct vm_page *vm_object_get_page(
 	struct vm_object *vo,
 	off_t offset,

vm/vm-object.c

@@ -278,6 +278,43 @@ extern struct vm_object *vm_object_create_in_place(
 	return vmo;
 }
 
+struct vm_object *vm_object_duplicate_cow(struct vm_object *vmo)
+{
+	struct object *obj = object_create(&vm_object_type);
+	if (!obj) {
+		return NULL;
+	}
+
+	struct vm_object *out = VM_OBJECT_CAST(obj);
+
+	memcpy(out->vo_name, vmo->vo_name, sizeof out->vo_name);
+	out->vo_flags = vmo->vo_flags;
+	out->vo_ctrl = vmo->vo_ctrl;
+	out->vo_prot = vmo->vo_prot;
+	out->vo_size = vmo->vo_size;
+	memcpy(out->vo_name, vmo->vo_name, sizeof vmo->vo_name);
+
+	struct btree_node *cur = btree_first(&vmo->vo_pages);
+	while (cur) {
+		struct vm_page *pg
+			= BTREE_CONTAINER(struct vm_page, p_bnode, cur);
+		int x = atomic_fetch_add(&pg->p_cow_ref, 1);
+		if (x == 0) {
+			/* this is the first cow-reference for this page, so
+			 * the address-space it belongs to will need one too */
+			atomic_fetch_add(&pg->p_cow_ref, 1);
+		}
+
+		tracek("convert page %zx to cow %u",
+		       vm_page_get_paddr(pg),
+		       pg->p_cow_ref);
+
+		cur = btree_next(cur);
+	}
+
+	return out;
+}
+
 static struct vm_page *alloc_page(struct vm_object *vo, off_t offset)
 {
 	struct vm_page *page = NULL;