Re: [PATCH 12/34] mm: vmscan: do not reclaim from kswapd if there is any eligible zone

[Johannes Weiner]

On Fri, Jul 08, 2016 at 10:34:48AM +0100, Mel Gorman wrote:
> kswapd scans from highest to lowest for a zone that requires balancing.
> This was necessary when reclaim was per-zone to fairly age pages on lower
> zones.  Now that we are reclaiming on a per-node basis, any eligible zone
> can be used and pages will still be aged fairly.  This patch avoids
> reclaiming excessively unless buffer_heads are over the limit and it's
> necessary to reclaim from a higher zone than requested by the waker of
> kswapd to relieve low memory pressure.
> 
> [hillf.zj@xxxxxxxxxxxxxxx: Force kswapd reclaim no more than needed]
> Link: http://lkml.kernel.org/r/1466518566-30034-12-git-send-email-mgorman@xxxxxxxxxxxxxxxxxxx
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

One and a half observations:

> @@ -3144,31 +3144,39 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
>  
>  		sc.nr_reclaimed = 0;
>  
> -		/* Scan from the highest requested zone to dma */
> -		for (i = classzone_idx; i >= 0; i--) {
> -			zone = pgdat->node_zones + i;
> -			if (!populated_zone(zone))
> -				continue;
> -
> -			/*
> -			 * If the number of buffer_heads in the machine
> -			 * exceeds the maximum allowed level and this node
> -			 * has a highmem zone, force kswapd to reclaim from
> -			 * it to relieve lowmem pressure.
> -			 */
> -			if (buffer_heads_over_limit && is_highmem_idx(i)) {
> -				classzone_idx = i;
> -				break;
> -			}
> +		/*
> +		 * If the number of buffer_heads in the machine exceeds the
> +		 * maximum allowed level then reclaim from all zones. This is
> +		 * not specific to highmem as highmem may not exist but it is
> +		 * it is expected that buffer_heads are stripped in writeback.

The mention of highmem in this comment make only sense within the
context of this diff; it'll be pretty confusing in the standalone
code.

Also, double "it is" :)