Re: [PATCH v9 06/14] mm: multi-gen LRU: minimal implementation

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

 +
> +static long get_nr_evictable(struct lruvec *lruvec, unsigned long max_seq,
> +			     unsigned long *min_seq, bool can_swap, bool *need_aging)
> +{
> +	int gen, type, zone;
> +	long old = 0;
> +	long young = 0;
> +	long total = 0;
> +	struct lru_gen_struct *lrugen = &lruvec->lrugen;
> +
> +	for (type = !can_swap; type < ANON_AND_FILE; type++) {
> +		unsigned long seq;
> +
> +		for (seq = min_seq[type]; seq <= max_seq; seq++) {
> +			long size = 0;
> +
> +			gen = lru_gen_from_seq(seq);
> +
> +			for (zone = 0; zone < MAX_NR_ZONES; zone++)
> +				size += READ_ONCE(lrugen->nr_pages[gen][type][zone]);
> +
> +			total += size;
> +			if (seq == max_seq)
> +				young += size;
> +			if (seq + MIN_NR_GENS == max_seq)
> +				old += size;
> +		}
> +	}
> +
> +	/* try to spread pages out across MIN_NR_GENS+1 generations */
> +	if (min_seq[LRU_GEN_FILE] + MIN_NR_GENS > max_seq)
> +		*need_aging = true;
> +	else if (min_seq[LRU_GEN_FILE] + MIN_NR_GENS < max_seq)
> +		*need_aging = false;

Can you explain/document the reason for the considering the below
conditions for ageing? 

> +	else if (young * MIN_NR_GENS > total)
> +		*need_aging = true;

Are we trying to consdier the case of more than half the total pages
young as needing ageing? If so should MIN_NR_GENS be 2 instead of using
that #define? Or 

> +	else if (old * (MIN_NR_GENS + 2) < total)
> +		*need_aging = true;

What is the significance of '+ 2' ? 

> +	else
> +		*need_aging = false;
> +
> +	return total > 0 ? total : 0;
> +}
> +