[PATCH RFC 06/15] mm/mglru: use explicit tier range in read_ctrl_pos()

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 or 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
half-open [tier_min, tier_max) interval, which is the conventional C
idiom.  The call sites become self-documenting:

  - get_tier_idx:   (0, 1) for tier 0, (tier, tier+1) for each tier
  - get_type_to_scan: (0, MAX_NR_TIERS) for the full range

No functional change.

Signed-off-by: Kairui Song <[email protected]>
---
 mm/vmscan.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index c2ea92c2b69e..a359d5a1ff41 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3192,8 +3192,8 @@ 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;
@@ -3202,7 +3202,7 @@ 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] +
@@ -4805,9 +4805,9 @@ 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);
+	read_ctrl_pos(lruvec, type, 0, 1, 2, &sp);
 	for (tier = 1; tier < MAX_NR_TIERS; tier++) {
-		read_ctrl_pos(lruvec, type, tier, 3, &pv);
+		read_ctrl_pos(lruvec, type, tier, tier + 1, 3, &pv);
 		if (!positive_ctrl_err(&sp, &pv))
 			break;
 	}
@@ -4828,8 +4828,8 @@ 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, 0, MAX_NR_TIERS, swappiness, &sp);
+	read_ctrl_pos(lruvec, LRU_GEN_FILE, 0, MAX_NR_TIERS, MAX_SWAPPINESS - swappiness, &pv);
 
 	return positive_ctrl_err(&sp, &pv);
 }

-- 
2.55.0