[RFC PATCH v4 09/16] mm/mglru: move folios from oldest gen to second-oldest gen from head to tail

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

For reclamation, it makes sense to reclaim folios from tail to
head, as folios near the head are relatively hot. However, when
moving folios from the oldest generation to the second-oldest
generation, using the tail-to-head order would effectively cause
a cold/hot inversion.

Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 mm/vmscan.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 27623d3bad95..17357b16d1a7 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -190,8 +190,20 @@ struct scan_control {
 			prefetchw(&prev->_field);			\
 		}							\
 	} while (0)
+#define prefetchw_next_lru_folio(_folio, _base, _field)			\
+	do {								\
+		if ((_folio)->lru.next != _base) {			\
+			struct folio *next;				\
+									\
+			next = list_entry((_folio)->lru.next,		\
+					struct folio, lru);		\
+			prefetchw(&next->_field);			\
+		}							\
+	} while (0)
+
 #else
 #define prefetchw_prev_lru_folio(_folio, _base, _field) do { } while (0)
+#define prefetchw_next_lru_folio(_folio, _base, _field) do { } while (0)
 #endif
 
 /*
@@ -3931,9 +3943,10 @@ static bool inc_min_seq(struct lruvec *lruvec, int type, int swappiness)
 	for (zone = 0; zone < MAX_NR_ZONES; zone++) {
 		struct list_head *head = &lrugen->folios[old_gen][type][zone];
 		unsigned long protected[MAX_NR_TIERS] = {}, delta = 0;
+		struct list_head *pos = head->next;
 
-		while (!list_empty(head)) {
-			struct folio *folio = lru_to_folio(head);
+		while (pos != head) {
+			struct folio *folio = list_entry(pos, struct folio, lru);
 			long nr_pages = folio_nr_pages(folio);
 			int refs = folio_lru_refs(folio);
 			bool workingset = folio_test_workingset(folio);
@@ -3944,6 +3957,8 @@ static bool inc_min_seq(struct lruvec *lruvec, int type, int swappiness)
 			VM_WARN_ON_ONCE_FOLIO(folio_is_file_lru(folio) != type, folio);
 			VM_WARN_ON_ONCE_FOLIO(folio_zonenum(folio) != zone, folio);
 
+			prefetchw_next_lru_folio(folio, head, flags);
+			pos = pos->next;
 			new_gen = __folio_inc_gen(folio, old_gen, &gen_increased);
 			if (gen_increased) {
 				delta += nr_pages;
-- 
2.34.1