[PATCH 7.3.y 1/2] mm/mglru: use explicit tier range in read_ctrl_pos()

[Sasha Levin <sashal@kernel.org>]

From: Kairui Song <kasong@tencent.com>

[ Upstream commit c1d0e799321c817a22eab2abf2485250cffad7b4 ]

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.

Link: https://lore.kernel.org/20260906-mglru-flags-cleanup-v6-5-9aacbd77d4ca@tencent.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Baoquan He <baoquan.he@linux.dev>
Reviewed-by: Barry Song <baohua@kernel.org>
Reviewed-by: Ridong Chen <ridong.chen@linux.dev>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: David Hildenbrand (Arm) <david@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <ryncsn@gmail.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lian Wang <lianux.mm@gmail.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Qi Zheng <qi.zheng@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Stable-dep-of: ca6e32f3f424 ("mm/vmscan: avoid false-positive -Wuninitialized warning, again")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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 94f9c3ff54160..da8dd04fb462a 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 963cdd67039d1..8ec8fdede5dd8 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3194,8 +3194,8 @@ 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;
@@ -3204,7 +3204,7 @@ 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] +
@@ -4808,9 +4808,9 @@ 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;
 	}
@@ -4831,8 +4831,10 @@ 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);
 }
-- 
2.53.0