kswapd endless loop for compaction

[Johannes Weiner <hannes@cmpxchg.org>]

Hi guys,

while testing a 3.7-rc5ish kernel, I noticed that kswapd can drop into
a busy spin state without doing reclaim.  printk-style debugging told
me that this happens when the distance between a zone's high watermark
and its low watermark is less than two huge pages (DMA zone).

1. The first loop in balance_pgdat() over the zones finds all zones to
be above their high watermark and only does goto out (all_zones_ok).

2. pgdat_balanced() at the out: label also just checks the high
watermark, so the node is considered balanced and the order is not
reduced.

3. In the `if (order)' block after it, compaction_suitable() checks if
the zone's low watermark + twice the huge page size is okay, which
it's not necessarily in a small zone, and so COMPACT_SKIPPED makes it
it go back to loop_again:.

This will go on until somebody else allocates and breaches the high
watermark and then hopefully goes on to reclaim the zone above low
watermark + 2 * THP.

I'm not really sure what the correct solution is.  Should we modify
the zone_watermark_ok() checks in balance_pgdat() to take into account
the higher watermark requirements for reclaim on behalf of compaction?
Change the check in compaction_suitable() / not use it directly?

Thanks,
Johannes

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