linux-kernel - [PATCH] mm:vmscan: fix shrink sc->nr counter values issue

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Date:   Wed, 29 Nov 2023 21:01:26 +0800
From:   Zhiguo Jiang <justinjiang@...o.com>
To:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     opensource.kernel@...o.com, Zhiguo Jiang <justinjiang@...o.com>
Subject: [PATCH] mm:vmscan: fix shrink sc->nr counter values issue

It is needed to ensure sc->nr.unqueued_dirty > 0, which can avoid to
set PGDAT_DIRTY flag when sc->nr.unqueued_dirty and sc->nr.file_taken
are both zero at the same time.

It can't be guaranteed for the PGDAT_WRITEBACK flag that only pages
marked for immediate reclaim are on evictable LRUs in other following
shrink processes of the same kswapd shrink recycling. So when both a
small amount of pages marked for immediate reclaim and a large amount
of pages marked for non-immediate reclaim are on evictable LRUs at the
same time, if it's only determined that there is at least a page marked
for immediate reclaim on evictable LRUs, kswapd shrink is throttled to
sleep, which will increase kswapd process consumption.

It can be fixed to throttle kswapd shrink when sc->nr.immediate is equal
to sc->nr.file_taken.

Signed-off-by: Zhiguo Jiang <justinjiang@...o.com>
---
 mm/vmscan.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)
 mode change 100644 => 100755 mm/vmscan.c

diff --git a/mm/vmscan.c b/mm/vmscan.c
index d8c3338fee0f..5723672bbdc2
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -5915,17 +5915,17 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 			set_bit(PGDAT_WRITEBACK, &pgdat->flags);

 		/* Allow kswapd to start writing pages during reclaim.*/
-		if (sc->nr.unqueued_dirty == sc->nr.file_taken)
+		if (sc->nr.unqueued_dirty && sc->nr.unqueued_dirty == sc->nr.file_taken)
 			set_bit(PGDAT_DIRTY, &pgdat->flags);

 		/*
-		 * If kswapd scans pages marked for immediate
+		 * If kswapd scans massive pages marked for immediate
 		 * reclaim and under writeback (nr_immediate), it
 		 * implies that pages are cycling through the LRU
 		 * faster than they are written so forcibly stall
 		 * until some pages complete writeback.
 		 */
-		if (sc->nr.immediate)
+		if (sc->nr.immediate && sc->nr.immediate == sc->nr.file_taken)
 			reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK);
 	}

-- 
2.39.0