Re: [PATCH 5/7] mm/mglru: use explicit tier range in read_ctrl_pos()

On 8/18/2026 1:38 PM, Kairui Song via B4 Relay wrote:
> From: Kairui Song <[email protected]>
> 
> read_ctrl_pos() encodes the tier range in a single "tier" parameter
> via "tier % MAX_NR_TIERS" as the start and "min(tier, MAX_NR_TIERS-1)"
> as the end. This is hard to follow, maintain, or extend. Tier values
> 0..3 select a single tier, while tier == MAX_NR_TIERS selects the
> full range.
> 
> Replace it with explicit (tier_min, tier_max) parameters using a
> closed [tier_min, tier_max] interval, and add LRU_TIER_MIN and
> LRU_TIER_MAX for the tier bounds. The call sites now become
> self-documenting:
> 
>    - get_tier_idx:   (LRU_TIER_MIN, LRU_TIER_MIN) for the first tier,
>                      (tier, tier) for each subsequent tier
>    - get_type_to_scan: (LRU_TIER_MIN, LRU_TIER_MAX) for the full range
> 
> No functional change.
> 

Just like LRU_GEN_MAX and MAX_NR_GENS issue.
Why do we have to add LRU_TIER_MAX, can we just use [tier_min, MAX_NR_TIERS)?


> Signed-off-by: Kairui Song <[email protected]>
> ---
>   include/linux/mmzone.h |  2 ++
>   mm/vmscan.c            | 18 ++++++++++--------
>   2 files changed, 12 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 32d9354a754f..d0b5c6217d25 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -492,6 +492,8 @@ enum lruvec_flags {
>    * folio->flags, masked by LRU_REFS_MASK.
>    */
>   #define MAX_NR_TIERS		4U
> +#define LRU_TIER_MIN		0U
> +#define LRU_TIER_MAX		(MAX_NR_TIERS - 1)
>   
>   #ifndef __GENERATING_BOUNDS_H
>   
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a819be6b7ae9..a613bb8d7271 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3198,8 +3198,8 @@ struct ctrl_pos {
>   	int gain;
>   };
>   
> -static void read_ctrl_pos(struct lruvec *lruvec, int type, int tier, int gain,
> -			  struct ctrl_pos *pos)
> +static void read_ctrl_pos(struct lruvec *lruvec, int type, int tier_min,
> +			  int tier_max, int gain, struct ctrl_pos *pos)
>   {
>   	int i;
>   	struct lru_gen_folio *lrugen = &lruvec->lrugen;
> @@ -3208,7 +3208,7 @@ static void read_ctrl_pos(struct lruvec *lruvec, int type, int tier, int gain,
>   	pos->gain = gain;
>   	pos->refaulted = pos->total = 0;
>   
> -	for (i = tier % MAX_NR_TIERS; i <= min(tier, MAX_NR_TIERS - 1); i++) {
> +	for (i = tier_min; i <= tier_max; i++) {
>   		pos->refaulted += lrugen->avg_refaulted[type][i] +
>   				  atomic_long_read(&lrugen->refaulted[hist][type][i]);
>   		pos->total += lrugen->avg_total[type][i] +
> @@ -4804,9 +4804,9 @@ static int get_tier_idx(struct lruvec *lruvec, int type)
>   	 * This value is chosen because any other tier would have at least twice
>   	 * as many refaults as the first tier.
>   	 */
> -	read_ctrl_pos(lruvec, type, 0, 2, &sp);
> -	for (tier = 1; tier < MAX_NR_TIERS; tier++) {
> -		read_ctrl_pos(lruvec, type, tier, 3, &pv);
> +	read_ctrl_pos(lruvec, type, LRU_TIER_MIN, LRU_TIER_MIN, 2, &sp);
> +	for (tier = LRU_TIER_MIN + 1; tier <= LRU_TIER_MAX; tier++) {
> +		read_ctrl_pos(lruvec, type, tier, tier, 3, &pv);
>   		if (!positive_ctrl_err(&sp, &pv))
>   			break;
>   	}
> @@ -4827,8 +4827,10 @@ static int get_type_to_scan(struct lruvec *lruvec, int swappiness)
>   	 * Compare the sum of all tiers of anon with that of file to determine
>   	 * which type to scan.
>   	 */
> -	read_ctrl_pos(lruvec, LRU_GEN_ANON, MAX_NR_TIERS, swappiness, &sp);
> -	read_ctrl_pos(lruvec, LRU_GEN_FILE, MAX_NR_TIERS, MAX_SWAPPINESS - swappiness, &pv);
> +	read_ctrl_pos(lruvec, LRU_GEN_ANON, LRU_TIER_MIN, LRU_TIER_MAX,
> +		      swappiness, &sp);
> +	read_ctrl_pos(lruvec, LRU_GEN_FILE, LRU_TIER_MIN, LRU_TIER_MAX,
> +		      MAX_SWAPPINESS - swappiness, &pv);
>   
>   	return positive_ctrl_err(&sp, &pv);
>   }
> 

-- 
Best regards
Ridong