Re: [PATCH] prevent kswapd from freeing excessive amounts of lowmem

[Rik van Riel]

Pavel Machek wrote:
> Hi!
>
> > The current VM can get itself into trouble fairly easily 
> > on systems
> > with a small ZONE_HIGHMEM, which is common on i686 
> > computers with
> > 1GB of memory.
> >
> > On one side, page_alloc() will allocate down to 
> > zone->pages_low,
> > while on the other side, kswapd() and balance_pgdat() 
> > will try
> > to free memory from every zone, until every zone has 
> > more free
> > pages than zone->pages_high.
> >
> > Highmem can be filled up to zone->pages_low with page 
> > tables,
> > ramfs, vmalloc allocations and other unswappable things 
> > quite
> > easily and without many bad side effects, since we still 
> > have
> > a huge ZONE_NORMAL to do future allocations from.
> >
> > However, as long as the number of free pages in the 
> > highmem
> > zone is below zone->pages_high, kswapd will continue 
> > swapping
> > things out from ZONE_NORMAL, too!
> >
> > Sami Farin managed to get his system into a stage where 
> > kswapd
> > had freed about 700MB of low memory and was still "going 
> > strong".
> >
> > The attached patch will make kswapd stop paging out data 
> > from
> > zones when there is more than enough memory free.  We do 
> > go above
> > zone->pages_high in order to keep pressure between zones 
> > equal
> > in normal circumstances, but the patch should prevent 
> > the kind
> > of excesses that made Sami's computer totally unusable.
> >
> > Please merge this into -mm.
> >
> > Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
>
> > --- linux-2.6.22.noarch/mm/vmscan.c.excessive	2007-09-05 12:19:49.000000000 -0400
> > +++ linux-2.6.22.noarch/mm/vmscan.c	2007-09-05 12:21:40.000000000 -0400
> > @@ -1371,7 +1371,13 @@ loop_again:
> >  			temp_priority[i] = priority;
> >  			sc.nr_scanned = 0;
> >  			note_zone_scanning_priority(zone, priority);
> > -			nr_reclaimed += shrink_zone(priority, zone, &sc);
> > +			/*
> > +			 * We put equal pressure on every zone, unless one
> > +			 * zone has way too many pages free already.
> > +			 */
>
> That does not seem right. Having empty HIGHMEM and full LOWMEM would
> be very bad, right? We may stop freeing when there's enough LOWMEM
> free, but not if there's only HIGHMEM free.

Please read the code this patch applies to.

The check I add conditionalizes the individual
calls to shrink_zone(), so we do not call
shrink_zone() for a zone that has a ton of free
pages.  We still call shrink_zone() for the
other zones.

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