linux-kernel - [PATCH v7 6/7] mm: vmscan: Avoid split during shrink_folio

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Date: Mon,  8 Apr 2024 19:39:45 +0100
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>,
	Lance Yang <ioworker0@...il.com>
Cc: Ryan Roberts <ryan.roberts@....com>,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Barry Song <v-songbaohua@...o.com>
Subject: [PATCH v7 6/7] 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.

THP_SWPOUT and THP_SWPOUT_FALLBACK counters should continue to count
events only for PMD-mappable folios to avoid user confusion. THP_SWPOUT
already has the appropriate guard. Add a guard for THP_SWPOUT_FALLBACK.
It may be appropriate to add per-size counters in future.

Reviewed-by: David Hildenbrand <david@...hat.com>
Reviewed-by: Barry Song <v-songbaohua@...o.com>
Signed-off-by: Ryan Roberts <ryan.roberts@....com>
---
 mm/vmscan.c | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 00adaf1cb2c3..bca2d9981c95 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1223,25 +1223,25 @@ 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 right away.
+					 * We can free the unmapped pages without IO.
 					 */
-					if (!folio_entire_mapcount(folio) &&
-					    split_folio_to_list(folio,
-								folio_list))
+					if (data_race(!list_empty(&folio->_deferred_list)) &&
+					    split_folio_to_list(folio, folio_list))
 						goto activate_locked;
 				}
 				if (!add_to_swap(folio)) {
 					if (!folio_test_large(folio))
 						goto activate_locked_split;
 					/* Fallback to swap normal pages */
-					if (split_folio_to_list(folio,
-								folio_list))
+					if (split_folio_to_list(folio, folio_list))
 						goto activate_locked;
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
-					count_memcg_folio_events(folio, THP_SWPOUT_FALLBACK, 1);
-					count_vm_event(THP_SWPOUT_FALLBACK);
+					if (nr_pages >= HPAGE_PMD_NR) {
+						count_memcg_folio_events(folio,
+							THP_SWPOUT_FALLBACK, 1);
+						count_vm_event(THP_SWPOUT_FALLBACK);
+					}
 #endif
 					if (!add_to_swap(folio))
 						goto activate_locked_split;
-- 
2.25.1