Linux: PG_pinned vs. PG_foreign

Linux: PG_pinned vs. PG_foreign

[Jan Beulich]

Is it intentional that these two both use PG_arch_1? It seems at least risky
to me... And if intended, it would certainly deserve a comment. (I was about
to utilize PG_pinned for indicating pinned highmem-allocated PTEs when I
realized this collision.)

Thanks, Jan

Re: Linux: PG_pinned vs. PG_foreign

[Keir Fraser]

On 15/1/07 09:38, "Jan Beulich" <jbeulich@novell.com> wrote:

> Is it intentional that these two both use PG_arch_1? It seems at least risky
> to me... And if intended, it would certainly deserve a comment. (I was about
> to utilize PG_pinned for indicating pinned highmem-allocated PTEs when I
> realized this collision.)

There's no reason to have them use the same bit if there's a PG_arch_2
available. It was probbaly laziness on my part when I realised that (so far)
ForeignPage and Pinned are mutually exclusive.

 -- Keir

Re: Linux: PG_pinned vs. PG_foreign

[Jan Beulich]

>>> Keir Fraser <keir@xensource.com> 15.01.07 11:10 >>>
>On 15/1/07 09:38, "Jan Beulich" <jbeulich@novell.com> wrote:
>
>> Is it intentional that these two both use PG_arch_1? It seems at least risky
>> to me... And if intended, it would certainly deserve a comment. (I was about
>> to utilize PG_pinned for indicating pinned highmem-allocated PTEs when I
>> realized this collision.)
>
>There's no reason to have them use the same bit if there's a PG_arch_2
>available. It was probbaly laziness on my part when I realised that (so far)
>ForeignPage and Pinned are mutually exclusive.

There isn't, but there are a few bits left, so that shouldn't be a problem.
PG_foreign really should be a standalone one, not using PR_arch_1, as
arches may have or get a meaning assigned for this (ia64 specifically has),
so even if it isn't a problem today it is very much like a latent bug.

I'll try to send a respective patch soon, but since I want to use this in our
code, too, I first have to resolve a collision with s390 patches from IBM,
which consume all the remaining bit positions.

Jan