Re: Status Bits and Super Pages

[Marcus Brinkmann]

At Thu, 23 Aug 2007 13:20:00 +0200,
Neal H. Walfield wrote:
> 
> Hi,
> 
> I'm writing a memory manager and in order to make eviction decisions,
> I want to use the status bits.  Because I'm a good citizen, I've
> written my memory manager to get the largest mappings possible from
> sigma0.  My resource manager, however, sub-divides these pages and
> maps them to clients.
> 
> I suspect (but please confirm) that my resource manager is only able
> to get the status bits for the mappings that it received from its
> pager, not the mappings it sent to its clients.  That is, if it got a
> 64MB fpage from sigma0, even though it may have only handed out 4KB
> pages to clients, there is only a single status bit available to it:
> the one for the 64MB page.  Is this interpretation correct?

As far as I remember it, yes, this is correct.
 
> If so, it seems that if I want status bits for the pages that the
> resource manager maps to its clients, then I have to be careful to
> only map pages of the same size as the ones the manager has.  I
> imagine that I can request new mappings from sigma0 as appropriate,

Or insert an intermediate task.

> but this basically defeats the advantage of having mapped the
> superpages to begin with--most mappings will be small and thus the
> address space with quickly be filled with small mappings.  Plus, with
> this approach, there is the overhead of the cost of the unmap and
> re-map operations.

Yes.  However, there is a related problem that you can not unmap
selectively.  I vaguely remember that Espen had a selective unmap
proposal at some time as a small modification to the L4 X.2 interface.
But I don't remember any details.  Whatever that proposal was should
also provide an opportunity to do selective status bit checking,
shouldn't it?

Thanks,
Marcus