Re: [RFC 12/13] mm, compaction: more reliably increase direct compaction priority

[Vlastimil Babka]

On 05/16/2016 10:14 AM, Michal Hocko wrote:
> On Mon 16-05-16 09:31:44, Vlastimil Babka wrote:
> > Yeah it should work, my only worry was that this may get subtly wrong (as
> > experience shows us) and due to e.g. slightly different watermark checks
> > and/or a corner-case zone such as ZONE_DMA, should_reclaim_retry() would
> > keep returning true, even if reclaim couldn't/wouldn't help anything. Then
> > compaction would be needlessly kept at ineffective priority.
>
> watermark check for ZONE_DMA should always fail because it fails even
> when is completely free to the lowmem reserves. I had a subtle bug in
> the original code to check highzone_idx rather than classzone_idx but
> that should the fix has been posted recently:
> http://lkml.kernel.org/r/1463051677-29418-2-git-send-email-mhocko@xxxxxxxxxx

Sure, but that just adds to the experience of being subtly wrong in this 
area :) But sure we can leave this part alone until proven wrong, I 
don't insist strongly.

> > Also my understanding of the initial compaction priorities is to lower the
> > latency if fragmentation is just light and there's enough memory. Once we
> > start struggling, I don't see much point in not switching to the full
> > compaction priority quickly.
>
> That is true but why to compact when there are high order pages and they
> are just hidden by the watermark check.

Compaction should skip such zone regardless of priority.