Subject: Re: Should ppc32 use CONFIG_HIGHPTE or not?

From: Benjamin Herrenschmidt <[email-masked]>
Date: Sat, 02 May 2009 06:14:07 +1000

On Fri, 2009-05-01 at 11:37 -0700, Dave Hansen wrote:

 .../...

> So, it looks like ppc32 never actually allocates highmem pte pages, but
> it *does* go to the trouble of at least trying to kmap_atomic() them.
> Should we just give ppc32 unconditional direct-mapped ptes?  Or, should
> we remove that #ifdef and let it allocate them in highmem when it can
> since we also have the code to support that?

We actually noticed that recently :-)

We implemented HIGHPTE support a long time ago, and then somebody
disabled HIGHPTE for both x86 and powerpc on the ground that it wasn't
reliable, I don't remember off hand who, I think it was in the 2.5.x
timeframe, and while it got re-enabled on x86 it never was on powerpc
(maybe because we never noticed it was disabled in the first place ;-)

Now, recently, some changes went in that could possibly be problematic
with HIGHPTE, at least I have a vague recollection of that, I think
Kumar was involved... Kumar, was this fixed ?

So depending on that, maybe we could revive the option ... or just
get rid of that HIGHPTE support and be done with it.

Cheers,
Ben.