[PATCH mm-new] mm/mglru: clarify the generation placement in lru_gen_folio_seq()

[Ridong Chen <ridong.chen@linux.dev>]

From: Ridong Chen <chenridong@xiaomi.com>

lru_gen_folio_seq() computes how many generations below max_seq a
folio should be added to, but stores it in a variable named "gen".
Elsewhere in MGLRU "gen" means an index into lrugen->folios[], i.e.
seq % MAX_NR_GENS, so reusing the name for a relative offset is
confusing.

The comment table above the code also no longer matches it. It only
covers the PG_active and the final branch, leaving out the reclaiming
and "can't evict yet" cases, and it lists LRU_REFS_FLAGS as an input
although lru_gen_folio_seq() never looks at it.

Rename the local variable to "age", redraw the table so that each
branch maps to the age and seq it produces, and note that the result
is clamped to min_seq[type].

No functional change.

Assisted-by: LLM
Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
---
 include/linux/mm_inline.h | 48 ++++++++++++++++++++++++++-------------
 1 file changed, 32 insertions(+), 16 deletions(-)

diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
index ab69b9930893..a933d8c14558 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -302,34 +302,50 @@ static inline unsigned long lru_gen_folio_seq(const struct lruvec *lruvec,
 					      const struct folio *folio,
 					      bool reclaiming)
 {
-	int gen;
+	int age;
 	int type = folio_is_file_lru(folio);
 	const struct lru_gen_folio *lrugen = &lruvec->lrugen;
 
 	/*
-	 * +-----------------------------------+-----------------------------------+
-	 * | Accessed through page tables and  | Accessed through file descriptors |
-	 * | promoted by folio_update_gen()    | and protected by folio_inc_gen()  |
-	 * +-----------------------------------+-----------------------------------+
-	 * | PG_active (set while isolated)    |                                   |
-	 * +-----------------+-----------------+-----------------+-----------------+
-	 * |  PG_workingset  |  PG_referenced  |  PG_workingset  |  LRU_REFS_FLAGS |
-	 * +-----------------------------------+-----------------------------------+
-	 * |<---------- MIN_NR_GENS ---------->|                                   |
-	 * |<---------------------------- MAX_NR_GENS ---------------------------->|
+	 *            +-----------------+-----------------+-----------------+-----------------+
+	 * age        |        1        |        2        |        3        |        4        |
+	 *            +-----------------+-----------------+-----------------+-----------------+
+	 * seq        |     max_seq     |    max_seq-1    |    max_seq-2    |    max_seq-3    |
+	 *            +-----------------------------------+-----------------------------------+
+	 *            |<---------- MIN_NR_GENS ---------->|                                   |
+	 *            |<---------------------------- MAX_NR_GENS ---------------------------->|
+	 *            +-----------------------------------+-----------------------------------+
+	 *            | Accessed through page tables and  | Accessed through file descriptors |
+	 *            | promoted by folio_update_gen()    | and protected by folio_inc_gen()  |
+	 *            +-----------------+-----------------+-----------------+-----------------+
+	 * PG_active  |  PG_workingset  | !PG_workingset  |                 |                 |
+	 *            +-----------------+-----------------+-----------------+-----------------+
+	 * reclaiming |                 |                 |                 |     always      |
+	 *            +-----------------+-----------------+-----------------+-----------------+
+	 * can't      |                 |  anon && not in |                 |                 |
+	 * evict yet  |                 |  swapcache, or  |                 |                 |
+	 *            |                 |  PG_reclaim &&  |                 |                 |
+	 *            |                 | dirty/writeback |                 |                 |
+	 *            +-----------------+-----------------+-----------------+-----------------+
+	 * else       |                 |                 |  PG_workingset  | !PG_workingset  |
+	 *            |                 |                 |       or        |       &&        |
+	 *            |                 |                 |  PG_referenced  | !PG_referenced  |
+	 *            +-----------------+-----------------+-----------------+-----------------+
+	 *
+	 * The resulting seq is clamped to min_seq[type].
 	 */
 	if (folio_test_active(folio))
-		gen = MIN_NR_GENS - folio_test_workingset(folio);
+		age = MIN_NR_GENS - folio_test_workingset(folio);
 	else if (reclaiming)
-		gen = MAX_NR_GENS;
+		age = MAX_NR_GENS;
 	else if ((!folio_is_file_lru(folio) && !folio_test_swapcache(folio)) ||
 		 (folio_test_reclaim(folio) &&
 		  (folio_test_dirty(folio) || folio_test_writeback(folio))))
-		gen = MIN_NR_GENS;
+		age = MIN_NR_GENS;
 	else
-		gen = MAX_NR_GENS - (folio_test_workingset(folio) || folio_test_referenced(folio));
+		age = MAX_NR_GENS - (folio_test_workingset(folio) || folio_test_referenced(folio));
 
-	return max(READ_ONCE(lrugen->max_seq) - gen + 1, READ_ONCE(lrugen->min_seq[type]));
+	return max(READ_ONCE(lrugen->max_seq) - age + 1, READ_ONCE(lrugen->min_seq[type]));
 }
 
 static inline bool lru_gen_add_folio(struct lruvec *lruvec, struct folio *folio, bool reclaiming)
-- 
2.34.1