Re: [patch] page aging and deferred swapping for 2.4.0-test1

[Rik van Riel <riel@conectiva.com.br>]

On Fri, 26 May 2000, Roger Larsson wrote:
> Rik van Riel wrote:

> > the attached patch attempts to implement the following two
> > things (which we'll probably want in the active/inactive
> > design later on):
> > - page aging (for active pages)
> > - deferred swap IO, with only unmapping in try_to_swap_out()

> The aging code can not be correct.
> 		if (PageTestandClearReferenced(page)) {
> 			page->age += 3;
> 			if (page->age > 10)
> 				page->age = 0;
> 			goto dispose_continue;
> 		}
> 		page->age--;
> 
> 		if (page->age)
> 			goto dispose_continue;

True, there is one obvious error here...

> I would say it should be:
> 
> 		if (PageTestandClearReferenced(page)) {
> 			page->age += 3;
> 			if (page->age > 10)
> 				page->age = 10;
> 			goto dispose_continue;
> 		}
> 
> 		if (page->age && priority)  // at zero priority ignore age
> 			goto dispose_continue;
> 
> 		page->age--;

This is wrong too. It would mean that we'd never decrease the
page age unless priority == 0 ;)

The fix is this:

}
-	page->age--;
+	if (page->age)
+		page->age--;


(so we cannot get into a near-infinite loop when we decrease
the unsigned age when it's zero and have it wrap to infinite)

regards,

Rik
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