Function vm_object_deallocate_anon

freebsd/vm/vm_object.c:565–604

* We dropped a reference on an object and discovered that it had a * single remaining shadow. This is a sibling of the reference we * dropped. Attempt to collapse the sibling and backing object. */

 * dropped.  Attempt to collapse the sibling and backing object.
 */
static vm_object_t
vm_object_deallocate_anon(vm_object_t backing_object)
{
	vm_object_t object;

	/* Fetch the final shadow.  */
	object = LIST_FIRST(&backing_object->shadow_head);
	KASSERT(object != NULL && backing_object->shadow_count == 1,
	    ("vm_object_anon_deallocate: ref_count: %d, shadow_count: %d",
	    backing_object->ref_count, backing_object->shadow_count));
	KASSERT((object->flags & (OBJ_TMPFS_NODE | OBJ_ANON)) == OBJ_ANON,
	    ("invalid shadow object %p", object));

	if (!VM_OBJECT_TRYWLOCK(object)) {
		/*
		 * Prevent object from disappearing since we do not have a
		 * reference.
		 */
		vm_object_pip_add(object, 1);
		VM_OBJECT_WUNLOCK(backing_object);
		VM_OBJECT_WLOCK(object);
		vm_object_pip_wakeup(object);
	} else
		VM_OBJECT_WUNLOCK(backing_object);

	/*
	 * Check for a collapse/terminate race with the last reference holder.
	 */
	if ((object->flags & (OBJ_DEAD | OBJ_COLLAPSING)) != 0 ||
	    !refcount_acquire_if_not_zero(&object->ref_count)) {
		VM_OBJECT_WUNLOCK(object);
		return (NULL);
	}
	backing_object = object->backing_object;
	if (backing_object != NULL && (backing_object->flags & OBJ_ANON) != 0)
		vm_object_collapse(object);
	VM_OBJECT_WUNLOCK(object);

	return (object);
}

/*
 *	vm_object_deallocate: