Re: [RFC PATCH 1/2] mm, vmscan: consider eligible zones in get_scan_count

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Jan 10, 2017 at 01:55:51PM +0100, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> get_scan_count considers the whole node LRU size when
> - doing SCAN_FILE due to many page cache inactive pages
> - calculating the number of pages to scan
> 
> in both cases this might lead to unexpected behavior especially on 32b
> systems where we can expect lowmem memory pressure very often.

The amount of retrofitting zones back into reclaim is disappointing :/

>  /*
> + * Return the number of pages on the given lru which are eligible for the
> + * given zone_idx
> + */
> +static unsigned long lruvec_lru_size_eligibe_zones(struct lruvec *lruvec,
> +		enum lru_list lru, int zone_idx)
> +{
> +	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
> +	unsigned long lru_size;
> +	int zid;
> +
> +	lru_size = lruvec_lru_size(lruvec, lru);
> +	for (zid = zone_idx + 1; zid < MAX_NR_ZONES; zid++) {
> +		struct zone *zone = &pgdat->node_zones[zid];
> +		unsigned long size;
> +
> +		if (!managed_zone(zone))
> +			continue;
> +
> +		size = lruvec_zone_lru_size(lruvec, lru, zid);
> +		lru_size -= min(size, lru_size);
> +	}
> +
> +	return lru_size;

The only other use of lruvec_lru_size() is also in get_scan_count(),
where it decays the LRU pressure balancing ratios. That caller wants
to operate on the entire lruvec.

Can you instead add the filtering logic to lruvec_lru_size() directly,
and pass MAX_NR_ZONES when operating on the entire lruvec? That would
make the code quite a bit clearer than having 3 different lruvec size
querying functions.

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