Re: Restricted storage

[Lluis]

El Mon, May 29, 2006 at 06:28:02PM +0200, Pierre THIERRY ens deleità amb les 
següents paraules:
> Scribit Bas Wijnen dies 29/05/2006 hora 14:28:
>>> I wonder if a write notice flag could be interesting. 
>> Perhaps.  I suspect that it will cost performance in situations where 
>> it isn't used though, which is unacceptable IMO (because it will 
>> hardly ever be used, I suppose).
> 
> I'm pretty sure it can be built so that when not used, it has no impact. 
> The notifier could be some sort of proxy, and it then it would just be 
> absent when not used. In most cases, write notification would only be 
> needed once (just to know when initial integrity is broken), and the 
> system could be made to remove that proxy then.

As of what I know, a proxy could only play a role when a page fault occurs, 
so it wouldn't be able to proxy _all_ accesses (and thus not all writes).

A compromise solution, which doesn't penalise the common case (no notice), 
would be to use pages that are always on COW.

Although it would have a bad performance when used (but no zero penalty 
when not used), it allows to capture all write accesses to the page and let 
a "proxy" send the notice.

Read you,
  Lluis

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