[RFC PATCH v3 1/6] mm: mglru: prevent min_seq[type] from pointing to an empty generation

["Barry Song (Xiaomi)" <baohua@kernel.org>]

In try_to_inc_min_seq(), min_seq[LRU_GEN_ANON] and
min_seq[LRU_GEN_FILE] can be adjusted to point to empty generations
to keep their gap within one. As a result, scan_folios() may
repeatedly scan empty generations because it assumes the min_seq
generation still contains folios. Likewise,
should_run_aging() has no way to tell that the min_seq generation
has already been drained.

This is quite confusing. I observed scan_folios() returning 0 even
though the following check in scan_folios() doesn't take effect:

        if (get_nr_gens(lruvec, type) == MIN_NR_GENS)
                return 0;

There is no need to adjust min_seq[], since the reclaim logic already
triggers aging when the number of generations reaches
MIN_NR_GENS, and reclaim never reduces it below MIN_NR_GENS.

Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 include/linux/mmzone.h |  6 ++----
 mm/vmscan.c            | 10 ----------
 2 files changed, 2 insertions(+), 14 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index a26c8b855222..233d2006a541 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -552,10 +552,8 @@ enum {
  * The youngest generation number is stored in max_seq for both anon and file
  * types as they are aged on an equal footing. The oldest generation numbers are
  * stored in min_seq[] separately for anon and file types so that they can be
- * incremented independently. Ideally min_seq[] are kept in sync when both anon
- * and file types are evictable. However, to adapt to situations like extreme
- * swappiness, they are allowed to be out of sync by at most
- * MAX_NR_GENS-MIN_NR_GENS-1.
+ * incremented independently. For both file and anonymous memory, the minimum
+ * generation must be at least MIN_NR_GENS.
  *
  * The number of pages in each generation is eventually consistent and therefore
  * can be transiently negative when reset_batch_size() is pending.
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 566c4e837c7d..ce027c271e9b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3972,16 +3972,6 @@ static void try_to_inc_min_seq(struct lruvec *lruvec, int swappiness)
 	if (!seq_inc_flag)
 		return;
 
-	/* see the comment on lru_gen_folio */
-	if (swappiness && swappiness <= MAX_SWAPPINESS) {
-		unsigned long seq = lrugen->max_seq - MIN_NR_GENS;
-
-		if (min_seq[LRU_GEN_ANON] > seq && min_seq[LRU_GEN_FILE] < seq)
-			min_seq[LRU_GEN_ANON] = seq;
-		else if (min_seq[LRU_GEN_FILE] > seq && min_seq[LRU_GEN_ANON] < seq)
-			min_seq[LRU_GEN_FILE] = seq;
-	}
-
 	for_each_evictable_type(type, swappiness) {
 		if (min_seq[type] <= lrugen->min_seq[type])
 			continue;
-- 
2.34.1