Re: 7-4 VM killing (A solution)

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

On Thu, 4 May 2000, Rajagopal Ananthanarayanan wrote:

> Ok, I may have a solution after having asked, mostly to myself,
> why doesn't shrink_mmap() find pages to free?
> 
> The answer apparenlty is because in 7-4 shrink_mmap(),
> unreferenced pages get filed as "young" if the zone has
> enough pages in it (free_pages > pages_high).
> 
> Because of this bug, if we examine a zone which already
> has enough free pages, all referenced pages now go to
> the "back" of the lru list.
> 
> On a subsequent scan, we may never get to these pages in time.
> Comments?
> 
> Here's the new code to shrink_mmap:
> 
> ------------
> 		[ ... ]
> 		 dispose = &young;
>                 if (test_and_clear_bit(PG_referenced, &page->flags))
>                         goto dispose_continue;
> 
>                 if (!page->buffers && page_count(page) > 1)
>                         goto dispose_continue;
> 
>                 dispose = &old;
>                 if (p_zone->free_pages > p_zone->pages_high)
>                         goto dispose_continue;

I've tried this variant (a few weeks ago, before submitting
the current code to Linus) and have found a serious bug in
it.

If we put all the unreferenced pages from one zone (with
enough free pages) on the front of the queue, a subsequent
run will not make it to the pages of the zone which needs
to have pages freed currently...

regards,

Rik
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