[RFC][POWERPC] Provide a way to protect 4k subpages when using 64k pages

[Arnd Bergmann]

On Tuesday 11 December 2007, Christoph Hellwig wrote:
> Using 4k pages should basically be a pre-process flag
> (which it already is as an implementation detail in your patch), and
> thus the proper way to mark it should be a personality flag.  This
> also means it could be implied by certain personalities, e.g. powerpc
> 32bit for full compatiblity.   All these process would use plain mmap/
> mprotect to deal with the subpage protections.

If you want to make it a per-process flag, wouldn't a prctl bit be more
appropriate than a personality flag? That way you could at least set
it independent from other personality settings.

> At least ia64 and mips have multiple pages sizes already and I suspect
> more architectures will grow support for it.

Another related option might be the alternative page table layouts that
Martin Schwidefsky has implemented for concurrent 2/3/4-level page tables
on s390.

	Arnd <><