Re: 2.4 and 2GB swap partition limit

[R.E.Wolff@BitWizard.nl (Rogier Wolff)]

Stephen C. Tweedie wrote:
> Hi,
> 
> On Tue, May 01, 2001 at 06:14:54PM +0200, Rogier Wolff wrote:
> 
> > Shouldn't the algorithm be: 
> > 
> > - If (current_access == write )
> > 	free (swap_page);
> >   else
> >  	map (page, READONLY)
> > 
> > and 
> >   when a write access happens, we fault again, and map free the 
> >   swap-page as it is now dirty anyway. 
> 
> That's what 2.2 did.  2.4 doesn't have to. 
> 
> The trouble is, you really want contiguous virtual memory to remain
> contiguous on swap.  Freeing individual pages like this on fault can
> cause a great deal of fragmentation in swap.  We'd far rather keep the
> swap page reserved for future use by the same page so that the VM
> region remains contiguous on disk.
> 
> That's fine as far as it goes, but the problem happens if you _never_
> free up such pages.  We should reap the unused swap page if we run out
> of swap.  We don't, and _that_ is the problem --- not the fact that
> the page is left allocated in the first place, but the fact that we
> don't do anything about it once we are short on disk.

first: Thanks for clearing this up for me. 

So, there are in fact some more "states" a swap-page can be in:

	-(0) free
	-(1) allocated, not in mem. 
	-(2) on swap, valid copy of memory. 
	-(3) on swap: invalid copy, allocated for fragmentation, can 
		be freed on demand if we are close to running out of swap.

If we running low on (0) swap-pages we can first start to reap the (3)
pages, and if that runs out, we can start reaping the (2)
pages. Right?


		Roger. 

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