Re: [PATCH] mm,vmscan: free pages if compaction_suitable tells us to

[Johannes Weiner]

On Sun, Nov 25, 2012 at 01:29:50PM -0500, Rik van Riel wrote:
> On Sun, 25 Nov 2012 17:57:28 +0100
> Johannes Hirte <johannes.hirte@xxxxxxxxxxxxxxxxx> wrote:
> 
> > With kernel 3.7-rc6 I've still problems with kswapd0 on my laptop
> 
> > And this is most of the time. I've only observed this behavior on the
> > laptop. Other systems don't show this.
> 
> This suggests it may have something to do with small memory zones,
> where we end up with the "funny" situation that the high watermark
> (+ balance gap) for a particular zone is less than the low watermark
> + 2<<order pages, which is the number of free pages required to keep
> compaction_suitable happy.
> 
> Could you try this patch?

It's not quite enough because it's not reaching the conditions you
changed, see analysis in https://lkml.org/lkml/2012/11/20/567

But even fixing it up (by adding the compaction_suitable() test in
this preliminary scan over the zones and setting end_zone accordingly)
is not enough because no actual reclaim happens at priority 12 in a
small zone.  So the number of free pages is not actually changing and
the compaction_suitable() checks keep the loop going.

The problem is fairly easy to reproduce, by the way.  Just boot with
mem=800M to have a relatively small lowmem reserve in the DMA zone.
Fill it up with page cache, then allocate transparent huge pages.

With your patch and my fix to the preliminary zone loop, there won't
be any hung task warnings anymore because kswapd actually calls
shrink_slab() and there is a rescheduling point in there, but it still
loops forever.

It also seems a bit aggressive to try to balance a small zone like DMA
for a huge page when it's not a GFP_DMA allocation, but none of these
checks actually take the classzone into account.  Do we have any
agreement over what this whole thing is supposed to be doing?

diff --git a/mm/vmscan.c b/mm/vmscan.c
index b99ecba..f7e54df 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2412,6 +2412,9 @@ static void age_active_anon(struct zone *zone, struct scan_control *sc)
  *     would need to be at least 256M for it to be balance a whole node.
  *     Similarly, on x86-64 the Normal zone would need to be at least 1G
  *     to balance a node on its own. These seemed like reasonable ratios.
+ *
+ * The kswapd source code is brought to you by Advil®.  "For today's
+ * tough pain, one might not be enough."
  */
 static bool pgdat_balanced(pg_data_t *pgdat, unsigned long balanced_pages,
 						int classzone_idx)
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