Function vm_page_acquire_unlocked

freebsd/vm/vm_page.c:4496–4541

* Locklessly attempt to acquire a page given a (object, pindex) tuple * and an optional previous page to avoid the radix lookup. The resulting * page will be validated against the identity tuple and busied or wired * as requested. A NULL *mp return guarantees that the page was not in * radix at the time of the call but callers must perform higher level * synchronization or retry the operati

 * (false, *mp == NULL) - WAITFAIL or NOWAIT prevented acquisition.
 */
static bool
vm_page_acquire_unlocked(vm_object_t object, vm_pindex_t pindex,
    vm_page_t prev, vm_page_t *mp, int allocflags)
{
	vm_page_t m;

	vm_page_grab_check(allocflags);
	MPASS(prev == NULL || vm_page_busied(prev) || vm_page_wired(prev));

	*mp = NULL;
	for (;;) {
		/*
		 * We may see a false NULL here because the previous page
		 * has been removed or just inserted and the list is loaded
		 * without barriers.  Switch to radix to verify.
		 */
		if (prev == NULL || (m = TAILQ_NEXT(prev, listq)) == NULL ||
		    QMD_IS_TRASHED(m) || m->pindex != pindex ||
		    atomic_load_ptr(&m->object) != object) {
			prev = NULL;
			/*
			 * This guarantees the result is instantaneously
			 * correct.
			 */
			m = vm_radix_lookup_unlocked(&object->rtree, pindex);
		}
		if (m == NULL)
			return (true);
		if (vm_page_trybusy(m, allocflags)) {
			if (m->object == object && m->pindex == pindex)
				break;
			/* relookup. */
			vm_page_busy_release(m);
			cpu_spinwait();
			continue;
		}
		if (!vm_page_grab_sleep(object, m, pindex, "pgnslp",
		    allocflags, false))
			return (false);
	}
	if ((allocflags & VM_ALLOC_WIRED) != 0)
		vm_page_wire(m);
	vm_page_grab_release(m, allocflags);
	*mp = m;
	return (true);
}

/*
 * Try to locklessly grab a page and fall back to the object lock if NOCREAT