Function vm_object_collapse

freebsd/vm/vm_object.c:1900–2045

* vm_object_collapse: * * Collapse an object with the object backing it. * Pages in the backing object are moved into the * parent, and the backing object is deallocated. */

 *	parent, and the backing object is deallocated.
 */
void
vm_object_collapse(vm_object_t object)
{
	vm_object_t backing_object, new_backing_object;

	VM_OBJECT_ASSERT_WLOCKED(object);

	while (TRUE) {
		KASSERT((object->flags & (OBJ_DEAD | OBJ_ANON)) == OBJ_ANON,
		    ("collapsing invalid object"));

		/*
		 * Wait for the backing_object to finish any pending
		 * collapse so that the caller sees the shortest possible
		 * shadow chain.
		 */
		backing_object = vm_object_backing_collapse_wait(object);
		if (backing_object == NULL)
			return;

		KASSERT(object->ref_count > 0 &&
		    object->ref_count > object->shadow_count,
		    ("collapse with invalid ref %d or shadow %d count.",
		    object->ref_count, object->shadow_count));
		KASSERT((backing_object->flags &
		    (OBJ_COLLAPSING | OBJ_DEAD)) == 0,
		    ("vm_object_collapse: Backing object already collapsing."));
		KASSERT((object->flags & (OBJ_COLLAPSING | OBJ_DEAD)) == 0,
		    ("vm_object_collapse: object is already collapsing."));

		/*
		 * We know that we can either collapse the backing object if
		 * the parent is the only reference to it, or (perhaps) have
		 * the parent bypass the object if the parent happens to shadow
		 * all the resident pages in the entire backing object.
		 */
		if (backing_object->ref_count == 1) {
			KASSERT(backing_object->shadow_count == 1,
			    ("vm_object_collapse: shadow_count: %d",
			    backing_object->shadow_count));
			vm_object_pip_add(object, 1);
			vm_object_set_flag(object, OBJ_COLLAPSING);
			vm_object_pip_add(backing_object, 1);
			vm_object_set_flag(backing_object, OBJ_DEAD);

			/*
			 * If there is exactly one reference to the backing
			 * object, we can collapse it into the parent.
			 */
			vm_object_collapse_scan(object);

#if VM_NRESERVLEVEL > 0
			/*
			 * Break any reservations from backing_object.
			 */
			if (__predict_false(!LIST_EMPTY(&backing_object->rvq)))
				vm_reserv_break_all(backing_object);
#endif