Re: [RFC v5 00/38] powerpc: Memory Protection Keys

From: Michal Hocko
Date: Wed Jul 12 2017 - 03:40:00 EST

On Wed 12-07-17 09:23:37, Michal Hocko wrote:
> On Tue 11-07-17 12:32:57, Ram Pai wrote:
> > Ideally the MMU looks at the PTE for keys, in order to enforce
> > protection. This is the case with x86 and is the case with power9 Radix
> > page table. Hence the keys have to be programmed into the PTE.
> But x86 doesn't update ptes for PKEYs, that would be just too expensive.
> You could use standard mprotect to do the same...

OK, this seems to be a misunderstanding and confusion on my end.
do_mprotect_pkey does mprotect_fixup even for the pkey path which is
quite surprising to me. I guess my misunderstanding comes from
Memory Protection Keys provides a mechanism for enforcing page-based
protections, but without requiring modification of the page tables
when an application changes protection domains. It works by
dedicating 4 previously ignored bits in each page table entry to a
"protection key", giving 16 possible keys.

So please disregard my previous comments about page tables and sorry
about the confusion.
Michal Hocko