Re: RFC: pageable kernel-segments

[Rik van Riel]

On Fri, 20 Apr 2001, Stephen C. Tweedie wrote:
> On Fri, Apr 20, 2001 at 03:49:30PM +0100, Alan Cox wrote:
> 
> > There is a proposal (several it seems) to make 2.5 replace the conventional
> > unix swap with a filesystem of backing store for anonymous objects. That will
> > mean each object has its own vm area and inode and thus we can start blowing
> > away all user mode page tables when we want.
> 
> Not without major VM overhaul.
> 
> The problem is MAP_PRIVATE, where a single vma can contain both normal
> file-backed pages and anonymous pages at the same time.  You don't
> even know whose anonymous page it is --- a process with anon pages can
> fork, so that later on some of the child's anon pages actually come
> from the parent's anon space instead of the child's.

Whoooops indeed. I forgot about this mess...

> Right now all of the magic that makes this work is in the page tables.
> To remove page tables we'd need additional structures all through the
> VM to track anonymous pages, and that's exactly where the FreeBSD VM
> starts to get extremely messy compared to ours.

That's because they still seem to use Mach's object chaining.

There's bound to be a much cleaner solution than whatever it
is they copied over from Mach ;)

regards,

Rik
--
Virtual memory is like a game you can't win;
However, without VM there's truly nothing to lose...

		http://www.surriel.com/
http://www.conectiva.com/	http://distro.conectiva.com.br/