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

[Hillf Danton]

On February 04, 2017 4:32 AM Mel Gorman wrote: 
> 
> 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@xxxxxxx>
> ---
Acked-by: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx>

>  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;
>  }
>