VM_FAULT_DIRTY
fault_flags = writing ? VM_FAULT_DIRTY : VM_FAULT_NORMAL;
(fs.fault_flags & (VM_FAULT_WIRE | VM_FAULT_DIRTY)) == 0) {
(fs->fault_flags & VM_FAULT_DIRTY) == 0) ||
(fs->fault_flags & VM_FAULT_DIRTY) != 0;