Re: Questions about copy-on-write

[Neal H. Walfield]

At Wed, 27 Oct 2004 15:14:16 -0400,
Rian Hunter wrote:
> yes definitely, physmem would have to know who has the page COW and who 
> really owns the page, otherwise how would it know who to make the 
> physical copy for on a fault.

This is wrong: if you have a frame with 10 users and one forces a copy
what happens?  Well obviously a copy is made and the copy is
associated with the task which wants to do that write which the other
nine continue to refer to the original.  The last one to do a write
gets the "original" frame--not the one who started the share.

> i think the general understanding has currently bin that 
> mapping/unmapping will be a privilegded operation. ie. only hurd servers 
> should be doing, or most precisely mostly physmem. untrusted tasks won't 
> really be unmapping/mapping, they will be using containers and closing 
> containers, and dying. physmem will take care of the actual business of 
> mapping and copying and etc.

It is only convention which says that that tasks do not create map
items or do grants.  Nothing else.  Tasks may do whatever they like.