linux-kernel - [PATCH 09/14] mm: deactivations shouldn't bias the LRU balance

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Date:   Wed, 20 May 2020 19:25:20 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     linux-mm@...ck.org
Cc:     Rik van Riel <riel@...riel.com>,
        Minchan Kim <minchan.kim@...il.com>,
        Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: [PATCH 09/14] mm: deactivations shouldn't bias the LRU balance

Operations like MADV_FREE, FADV_DONTNEED etc. currently move any
affected active pages to the inactive list to accelerate their reclaim
(good) but also steer page reclaim toward that LRU type, or away from
the other (bad).

The reason why this is undesirable is that such operations are not
part of the regular page aging cycle, and rather a fluke that doesn't
say much about the remaining pages on that list; they might all be in
heavy use, and once the chunk of easy victims has been purged, the VM
continues to apply elevated pressure on those remaining hot pages. The
other LRU, meanwhile, might have easily reclaimable pages, and there
was never a need to steer away from it in the first place.

As the previous patch outlined, we should focus on recording actually
observed cost to steer the balance rather than speculating about the
potential value of one LRU list over the other. In that spirit, leave
explicitely deactivated pages to the LRU algorithm to pick up, and let
rotations decide which list is the easiest to reclaim.

Signed-off-by: Johannes Weiner <hannes@...xchg.org>
Acked-by: Minchan Kim <minchan@...nel.org>
Acked-by: Michal Hocko <mhocko@...e.com>
---
 mm/swap.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/mm/swap.c b/mm/swap.c
index 5d62c5a0c651..d7912bfb597f 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -515,14 +515,12 @@ static void lru_deactivate_file_fn(struct page *page, struct lruvec *lruvec,

 	if (active)
 		__count_vm_event(PGDEACTIVATE);
-	lru_note_cost(lruvec, !file, hpage_nr_pages(page));
 }

 static void lru_deactivate_fn(struct page *page, struct lruvec *lruvec,
 			    void *arg)
 {
 	if (PageLRU(page) && PageActive(page) && !PageUnevictable(page)) {
-		int file = page_is_file_lru(page);
 		int lru = page_lru_base_type(page);

 		del_page_from_lru_list(page, lruvec, lru + LRU_ACTIVE);
@@ -531,7 +529,6 @@ static void lru_deactivate_fn(struct page *page, struct lruvec *lruvec,
 		add_page_to_lru_list(page, lruvec, lru);

 		__count_vm_events(PGDEACTIVATE, hpage_nr_pages(page));
-		lru_note_cost(lruvec, !file, hpage_nr_pages(page));
 	}
 }

@@ -556,7 +553,6 @@ static void lru_lazyfree_fn(struct page *page, struct lruvec *lruvec,

 		__count_vm_events(PGLAZYFREE, hpage_nr_pages(page));
 		count_memcg_page_event(page, PGLAZYFREE);
-		lru_note_cost(lruvec, 0, hpage_nr_pages(page));
 	}
 }

-- 
2.26.2