[RFC PATCH 3/4] mm/compaction: optimize >0 order folio compaction by sorting source pages.

[Zi Yan <zi.yan@sent.com>]

From: Zi Yan <ziy@nvidia.com>

It should maximize high order free page use and minimize free page splits.
It might be useful before free page merging is implemented.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 mm/compaction.c | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/mm/compaction.c b/mm/compaction.c
index 45747ab5f380..4300d877b824 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -145,6 +145,38 @@ static void sort_free_pages(struct list_head *src, struct free_list *dst)
 	}
 }
 
+static void sort_folios_by_order(struct list_head *pages)
+{
+	struct free_list page_list[MAX_ORDER + 1];
+	int order;
+	struct folio *folio, *next;
+
+	for (order = 0; order <= MAX_ORDER; order++) {
+		INIT_LIST_HEAD(&page_list[order].pages);
+		page_list[order].nr_free = 0;
+	}
+
+	list_for_each_entry_safe(folio, next, pages, lru) {
+		order = folio_order(folio);
+
+		if (order > MAX_ORDER)
+			continue;
+
+		list_move(&folio->lru, &page_list[order].pages);
+		page_list[order].nr_free++;
+	}
+
+	for (order = MAX_ORDER; order >= 0; order--) {
+		if (page_list[order].nr_free) {
+
+			list_for_each_entry_safe(folio, next,
+						 &page_list[order].pages, lru) {
+				list_move_tail(&folio->lru, pages);
+			}
+		}
+	}
+}
+
 #ifdef CONFIG_COMPACTION
 bool PageMovable(struct page *page)
 {
@@ -2636,6 +2668,8 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
 				pageblock_start_pfn(cc->migrate_pfn - 1));
 		}
 
+		sort_folios_by_order(&cc->migratepages);
+
 		err = migrate_pages(&cc->migratepages, compaction_alloc,
 				compaction_free, (unsigned long)cc, cc->mode,
 				MR_COMPACTION, &nr_succeeded);
-- 
2.40.1