Re: [PATCH 5/6] mm/mremap: Use pmd/pud_poplulate to update page table entries

On Sun, Jun 13, 2021 at 02:36:13PM +0530, Aneesh Kumar K.V wrote:

IIUC the reason why we do have pmd_pgtable() is that pgtable_t type
is arch dependent. On some architecture it is pte_t * and on the other
struct page *. The reason being highmem and level 4 page table can
be located in highmem. 

That is ahistorical.  See 2f569afd9ced9ebec9a6eb3dbf6f83429be0a7b4 --
we have pgtable_t for the benefit of s390's crazy sub-page page table
sizes.

Also, please stop numbering page tables upside down.  PTEs are first
level, not fourth.