linux-kernel - [PATCH v4 5/6] mm: vmscan: Avoid split during shrink_folio

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Date: Mon, 11 Mar 2024 15:00:57 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Andrew Morton <akpm@...ux-foundation.org>,
	David Hildenbrand <david@...hat.com>,
	Matthew Wilcox <willy@...radead.org>,
	Huang Ying <ying.huang@...el.com>,
	Gao Xiang <xiang@...nel.org>,
	Yu Zhao <yuzhao@...gle.com>,
	Yang Shi <shy828301@...il.com>,
	Michal Hocko <mhocko@...e.com>,
	Kefeng Wang <wangkefeng.wang@...wei.com>,
	Barry Song <21cnbao@...il.com>,
	Chris Li <chrisl@...nel.org>
Cc: Ryan Roberts <ryan.roberts@....com>,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v4 5/6] mm: vmscan: Avoid split during shrink_folio_list()

Now that swap supports storing all mTHP sizes, avoid splitting large
folios before swap-out. This benefits performance of the swap-out path
by eliding split_folio_to_list(), which is expensive, and also sets us
up for swapping in large folios in a future series.

If the folio is partially mapped, we continue to split it since we want
to avoid the extra IO overhead and storage of writing out pages
uneccessarily.

Signed-off-by: Ryan Roberts <ryan.roberts@....com>
---
 mm/vmscan.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index cf7d4cf47f1a..0ebec99e04c6 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1222,11 +1222,12 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 					if (!can_split_folio(folio, NULL))
 						goto activate_locked;
 					/*
-					 * Split folios without a PMD map right
-					 * away. Chances are some or all of the
-					 * tail pages can be freed without IO.
+					 * Split partially mapped folios map
+					 * right away. Chances are some or all
+					 * of the tail pages can be freed
+					 * without IO.
 					 */
-					if (!folio_entire_mapcount(folio) &&
+					if (!list_empty(&folio->_deferred_list) &&
 					    split_folio_to_list(folio,
 								folio_list))
 						goto activate_locked;
-- 
2.25.1