Re: Arch specific mmap attributes (Was: mprotect pgprot handling weirdness)

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Tue, 2010-04-06 at 19:26 +0900, KOSAKI Motohiro wrote:
> > Ok, I see. No biggie. The main deal remains how we want to do that
> > inside the kernel :-) I think the less horrible options here are
> > to either extend vm_flags to always be 64-bit, or add a separate
> > vm_map_attributes flag, and add the necessary bits and pieces to
> > prevent merge accross different attribute vma's.
> 
> vma->vm_flags already have VM_SAO. Why do we need more flags?
> At least, I dislike to add separate flags member into vma.
> It might introduce unnecessary messy into vma merge thing.

Well, we did shove SAO in there, and used up the very last vm_flag for
it a while back. Now I need another one, for little endian mappings. So
I'm stuck.

But the problem goes further I believe. Archs do nowadays have quite an
interesting set of MMU attributes that it would be useful to expose to
some extent.

Some powerpc's also provide storage keys for example and I think ARM
have something along those lines. There's interesting cachability
attributes too, on x86 as well. Being able to use such attributes to
request for example a relaxed ordering mapping on x86 might be useful.

I think it basically boils down to either extend vm_flags to always be
64-bit, which seems to be Nick preferred approach, or introduct a
vm_attributes with all the necessary changes to the merge code to take
it into account (not -that- hard tho, there's only half a page of
results in grep for these things :-)

Cheers,
Ben.