Re: [patch 1/6] mm: Allow architectures to define additional protection bits

On Mon, 2008-07-07 at 22:11 +0100, Hugh Dickins wrote:

Sorry, Andrew got the wrong pantomime: I was appearing in Aladdin
a couple of years ago, but this year I'm the Sleeping Beauty.
(Did I hear a grumble of dissent from the back stalls?)

No comment :-)

I don't find Dave's patch very handsome, but it gets the job done
so I'd better not carp.  The ugliness in vm_get_page_prot is just
an inevitable consequence of growing beyond the traditional neat
pairing of VM_xxx flags with VM_MAYxxx flags, along with the way
that opaque pgprot_t type becomes occasionally tiresome, as such
opaque types do: I don't think there's a better way of handling
it than Dave has done.

That was also my conclusion. It didn't look pretty but I couldn't come
up with something prettier.
 

There is a little inconsistency, that arch_calc_vm_prot_bits
and arch_vm_get_page_prot just handle the exceptional flag (SAO),
whereas arch_validate_prot handles all of them; but I don't feel
so strongly about that to suggest resubmission.

And regarding VM_SAO added to include/linux/mm.h in 3/6: although
it's odd to be weaving back and forth between arch-specific and
common, it's already the case that mman definitions and pgtable
definitions are arch-specific but mm.h common: I'm much happier
to have VM_SAO defined once there as Dave has it, than get into
arch-specific vm_flags.

Is someone going to be asking for PROT_WC shortly?

I'll definitely come with PROT_ENDIAN soon :-) (ie, some powerpc
processors can have a per-page endian flag that when set causes all
load/store instructions on this are to be byte-flipped, support for this
feature has been requested for some time, and now I have the
infrastructure to do it).

Cheers,
Ben.