[PATCH] mm, vmscan: Clear PGDAT_WRITEBACK when zone is balanced

From	Mel Gorman <mgorman@suse.de>
Newsgroups	linux.kernel
Subject	[PATCH] mm, vmscan: Clear PGDAT_WRITEBACK when zone is balanced
Date	2017-02-03 21:40 +0100
Message-ID	<t6SOe-6J7-11@gated-at.bofh.it> (permalink)
Organization	linux.* mail to news gateway

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Hillf Danton pointed out that since commit 1d82de618dd ("mm, vmscan:
make kswapd reclaim in terms of nodes") that PGDAT_WRITEBACK is no longer
cleared. It was not noticed as triggering it requires pages under writeback
to cycle twice through the LRU and before kswapd gets stalled. Historically,
such issues tended to occur on small machines writing heavily to slow
storage such as a USB stick. Once kswapd stalls, direct reclaim stalls may
be higher but due to the fact that memory pressure is requires, it would not
be very noticable. Michal Hocko suggested removing the flag entirely but
the conservative fix is to restore the intended PGDAT_WRITEBACK behaviour
and clear the flag when a suitable zone is balanced.

Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 mm/vmscan.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 532a2a750952..3379fa5ce6d8 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3103,6 +3103,7 @@ static bool zone_balanced(struct zone *zone, int order, int classzone_idx)
 	 */
 	clear_bit(PGDAT_CONGESTED, &zone->zone_pgdat->flags);
 	clear_bit(PGDAT_DIRTY, &zone->zone_pgdat->flags);
+	clear_bit(PGDAT_WRITEBACK, &zone->zone_pgdat->flags);
 
 	return true;
 }

-- 
Mel Gorman
SUSE Labs

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[PATCH] mm, vmscan: Clear PGDAT_WRITEBACK when zone is balanced Mel Gorman <mgorman@suse.de> - 2017-02-03 21:40 +0100
  Re: [PATCH] mm, vmscan: Clear PGDAT_WRITEBACK when zone is balanced Johannes Weiner <hannes@cmpxchg.org> - 2017-02-03 22:30 +0100
  Re: [PATCH] mm, vmscan: Clear PGDAT_WRITEBACK when zone is balanced "Hillf Danton" <hillf.zj@alibaba-inc.com> - 2017-02-04 04:20 +0100
  Re: [PATCH] mm, vmscan: Clear PGDAT_WRITEBACK when zone is balanced Michal Hocko <mhocko@kernel.org> - 2017-02-05 11:20 +0100
    Re: [PATCH] mm, vmscan: Clear PGDAT_WRITEBACK when zone is balanced Michal Hocko <mhocko@kernel.org> - 2017-02-05 11:20 +0100

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