Ralf Baechle wrote:

quoted

The code in pgtable-64.h assumes TASK_SIZE is always bigger than a first 
level PGDIR_SIZE. This is not the case for 64K pages, where task size is 
40 bits (1TB) and a pgd entry can map 42 bits. This leads to 
USER_PTRS_PER_PGD being zero for 64K pages.

Do you actually need large address space?  The kernel could allow 2, 3 and
4-level pagetables easily but doesn't give that choice currently.

Yes, in some cases. I'd like to benchmark the performance difference 
between 2 and 3 level pagetables, so the choice is valuable.