Re: Questions about copy-on-write

[Sam Mason]

Rian Hunter wrote:
>All of these questions will be answered in the current L4/Pistachio 
>reference manual. It is almost imperative that you understand how L4 is 
>doing things before worrying how the HURD works on top of it.

I've been trying to read through bits of it but it's not the most
approachable document in the world I've come across.

>In general L4 does hand out pages (only once) using a server called 
>"sigma0" which is where physmem takes all the memory, then takes over 
>managing the memory. By hand out I mean map to a task.

I figured that out quite early, but I guess its significance has faded
a bit with all the new things I've been finding out since then.

>A task revokes a mapping it gave by using the Unmap system call. This is 
>how physmem will revoke a mapping. A discussion that Neal and Marcus had 
>was how physmem will go about unmapping extra frames from user tasks, 
>either pre-emptively unmapping frames it designates as extra or giving 
>the task time to decide which one of it's frames it's extra then 
>conceding that frame. I think it's safe to say that tasks in HURD/L4 
>will not have to worry about using the Unmap system call.

That's been making sense for a few hours now. . .

>In the HURD I think it's convention that a user task will revoke a 
>mapping from another task by asking physmem to do it.

OK, is there any reason for that?  Is it anything to do with the
guaranteed page accounting process in physmem and if the task forgets
to tell physmem that it's not using a page anymore it'll be counted as
being used?

Cheers,
  Sam