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    Subject[PATCH 4.9 16/70] mm, vmscan: clear PGDAT_WRITEBACK when zone is balanced
    Date
    4.9-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Mel Gorman <mgorman@suse.de>

    commit c2f83143f1c67d186520b72b6cefbf0aa07a34ee upstream.

    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 required, 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.

    Fixes: 1d82de618ddd ("mm, vmscan: make kswapd reclaim in terms of nodes")
    Link: http://lkml.kernel.org/r/20170203203222.gq7hk66yc36lpgtb@suse.de
    Signed-off-by: Mel Gorman <mgorman@suse.de>
    Acked-by: Johannes Weiner <hannes@cmpxchg.org>
    Acked-by: Michal Hocko <mhocko@suse.com>
    Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
    Cc: Minchan Kim <minchan.kim@gmail.com>
    Cc: Rik van Riel <riel@redhat.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    mm/vmscan.c | 1 +
    1 file changed, 1 insertion(+)

    --- a/mm/vmscan.c
    +++ b/mm/vmscan.c
    @@ -3123,6 +3123,7 @@ static bool zone_balanced(struct zone *z
    */
    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;
    }

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