Re: [PATCH 6/7] mm: convert generic code to 5-level paging

From: Kirill A. Shutemov
Date: Wed Mar 08 2017 - 10:25:27 EST

On Wed, Mar 08, 2017 at 02:57:35PM +0100, Michal Hocko wrote:
> On Mon 06-03-17 23:45:13, Kirill A. Shutemov wrote:
> > Convert all non-architecture-specific code to 5-level paging.
> >
> > It's mostly mechanical adding handling one more page table level in
> > places where we deal with pud_t.
> >
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> OK, I haven't spotted anything major. I am just scratching my head about
> the __ARCH_HAS_5LEVEL_HACK leak into kasan_init.c (see below). Why do we
> need it? It looks more than ugly but I am not familiar with kasan so
> maybe this is really necessary.

Yeah ugly.

kasan_zero_p4d is only defined if we have real page table level. It's okay
if the page table level is folded properly -- using pgtable-nop4d.h -- in
this case pgd_populate() is nop and we don't reference kasan_zero_p4d.

With 5level-fixup.h, pgd_populate() is not nop, so we would reference
kasan_zero_p4d and build breaks. We don't need this as p4d_populate()
would do what we really need in this case.

We can drop the hack once all architectures that support kasan would be
converted to pgtable-nop4d.h -- amd64 and x86 at the moment.

Makes sense?

Kirill A. Shutemov