Function vm_object_shadow

freebsd/vm/vm_object.c:1417–1488

* vm_object_shadow: * * Create a new object which is backed by the * specified existing object range. The source * object reference is deallocated. * * The new object and offset into that object * are returned in the source parameters. */

 *	are returned in the source parameters.
 */
void
vm_object_shadow(vm_object_t *object, vm_ooffset_t *offset, vm_size_t length,
    struct ucred *cred, bool shared)
{
	vm_object_t source;
	vm_object_t result;

	source = *object;

	/*
	 * Don't create the new object if the old object isn't shared.
	 *
	 * If we hold the only reference we can guarantee that it won't
	 * increase while we have the map locked.  Otherwise the race is
	 * harmless and we will end up with an extra shadow object that
	 * will be collapsed later.
	 */
	if (source != NULL && source->ref_count == 1 &&
	    (source->flags & OBJ_ANON) != 0)
		return;

	/*
	 * Allocate a new object with the given length.
	 */
	result = vm_object_allocate_anon(atop(length), source, cred, length);

	/*
	 * Store the offset into the source object, and fix up the offset into
	 * the new object.
	 */
	result->backing_object_offset = *offset;

	if (shared || source != NULL) {
		VM_OBJECT_WLOCK(result);

		/*
		 * The new object shadows the source object, adding a
		 * reference to it.  Our caller changes his reference
		 * to point to the new object, removing a reference to
		 * the source object.  Net result: no change of
		 * reference count, unless the caller needs to add one
		 * more reference due to forking a shared map entry.
		 */
		if (shared) {
			vm_object_reference_locked(result);
			vm_object_clear_flag(result, OBJ_ONEMAPPING);
		}

		/*
		 * Try to optimize the result object's page color when
		 * shadowing in order to maintain page coloring
		 * consistency in the combined shadowed object.
		 */
		if (source != NULL) {
			vm_object_backing_insert(result, source);
			result->domain = source->domain;
#if VM_NRESERVLEVEL > 0
			result->flags |= source->flags & OBJ_COLORED;