Re: [PATCH 06/34] mm, vmscan: have kswapd only scan based on the highest requested zone

[Johannes Weiner]

On Fri, Jul 08, 2016 at 10:34:42AM +0100, Mel Gorman wrote:
> kswapd checks all eligible zones to see if they need balancing even if it
> was woken for a lower zone.  This made sense when we reclaimed on a
> per-zone basis because we wanted to shrink zones fairly so avoid
> age-inversion problems.  Ideally this is completely unnecessary when
> reclaiming on a per-node basis.  In theory, there may still be anomalies
> when all requests are for lower zones and very old pages are preserved in
> higher zones but this should be the exceptional case.
> 
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

I wasn't quite sure at first what the rationale is for this patch,
since it probably won't make much difference in pratice. But I do
agree that the code is cleaner to have kswapd check exactly what it
was asked to check, rather than some do-the-"right"-thing magic.

A hypothetical onslaught of low-zone allocations will wreak havoc to
the page age in higher zones anyway, right? So I don't think that case
matters all that much.