Re: [PATCH 11/27] mm: vmscan: Do not reclaim from kswapd if there is any eligible zone

[Vlastimil Babka]

On 06/09/2016 08:04 PM, 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.

Looks like the code was even wrong before... if classzone_idx wasn't 
already set to the highmem zone in the first place, it wouldn't look at it.

> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

After fixing the bug below,

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  mm/vmscan.c | 32 +++++++++++++++++++-------------
>  1 file changed, 19 insertions(+), 13 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index e4f3e068b7a0..6663fc75c3bc 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3102,24 +3102,30 @@ 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 */
> +		/*
> +		 * 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) {
> +			for (i = MAX_NR_ZONES - 1; i >= 0; i++) {

                                                           i--

> +				zone = pgdat->node_zones + i;
> +				if (!populated_zone(zone))
> +					continue;
> +
> +				if (is_highmem_idx(i))
> +					classzone_idx = i;
> +				break;
> +			}
> +		}
> +
> +		/* Only reclaim if there are no eligible zones */
>  		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 (!zone_balanced(zone, order, 0)) {
> +			if (!zone_balanced(zone, sc.order, classzone_idx)) {
>  				classzone_idx = i;
>  				break;
>  			}