Re: [Xen-devel] [PATCH] x86: fix preemptable page type handling

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 30/10/08 09:50, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

> - retain a page reference when PGT_partial is set on a page (and drop
>   it when clearing that flag)
> - don't drop a page reference never acquired when freeing the page type
>   of a page where the allocation of the type got preempted (and never
>   completed)
> - don't acquire a page reference when allocating the page type of a
>   page where freeing the type got preempted (and never completed, and
>   hence didn't drop the respective reference)

I don't understand exactly what's being explained here, so nobody else
really stands a chance. Reference counting (both general count and type
count) and its interaction with preemption is now complicated enough that it
needs an explanation from first principles. That should go both in the
changeset comment and in a code comment somewhere appropriate (I would say
in the comment in mm.h that your patch already modifies) -- it'll be a
rather annoyingly large code comment, but it will have significant value in
describing something that will otherwise make people's brains bleed. If you
can have a go at bashing something out that is basically comprehensive, I
can knock off any rough edges in the explanation before checking in.

 Thanks,
 Keir



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