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

From: Michal Hocko
Date: Tue Jul 11 2017 - 10:53:08 EST

On Wed 05-07-17 14:21:37, Ram Pai wrote:
> Memory protection keys enable applications to protect its
> address space from inadvertent access or corruption from
> itself.
> The overall idea:
> A process allocates a key and associates it with
> an address range within its address space.
> The process then can dynamically set read/write
> permissions on the key without involving the
> kernel. Any code that violates the permissions
> of the address space; as defined by its associated
> key, will receive a segmentation fault.
> This patch series enables the feature on PPC64 HPTE
> platform.
> ISA3.0 section 5.7.13 describes the detailed specifications.

Could you describe the highlevel design of this feature in the cover
letter. I have tried to get some idea from the patchset but it was
really far from trivial. Patches are not very well split up (many
helpers are added without their users etc..).

> Testing:
> This patch series has passed all the protection key
> tests available in the selftests directory.
> The tests are updated to work on both x86 and powerpc.
> version v5:
> (1) reverted back to the old design -- store the
> key in the pte, instead of bypassing it.
> The v4 design slowed down the hash page path.

This surprised me a lot but I couldn't find the respective code. Why do
you need to store anything in the pte? My understanding of PKEYs is that
the setup and teardown should be very cheap and so no page tables have
to updated. Or do I just misunderstand what you wrote here?
Michal Hocko