GUARD_SZ
KERN_VM_SZ - GUARD_SZ, apply_range_set_cb, NULL);
KERN_VM_SZ - GUARD_SZ, existing_page_cb, NULL);
#define KERN_VM_SZ (SZ_4G + GUARD_SZ)
return arena ? (u64) (long) arena->kern_vm->addr + GUARD_SZ / 2 : 0;