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Date: Sun, 5 Feb 2017 11:13:16 +0100
From: Michal Hocko <mhocko@kernel.org>
To: Mel Gorman <mgorman@suse.de>
Cc: "'Andrew Morton'" <akpm@linux-foundation.org>,
        Hillf Danton <hillf.zj@alibaba-inc.com>,
        "'Johannes Weiner'" <hannes@cmpxchg.org>,
        "'Minchan Kim'" <minchan.kim@gmail.com>,
        "'Rik van Riel'" <riel@redhat.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm, vmscan: Clear PGDAT_WRITEBACK when zone is balanced
Message-ID: <20170205101316.GC22713@dhcp22.suse.cz>
References: <20170203203222.gq7hk66yc36lpgtb@suse.de>
 <20170205101113.GB22713@dhcp22.suse.cz>
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On Sun 05-02-17 11:11:13, Michal Hocko wrote:
> On Fri 03-02-17 20:32:22, 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@suse.de>
> 
> I agree this is a more conservative approach but I think removing
> PGDAT_WRITEBACK should simplify things a bit.
> 
> Acked-by: Michal Hocko <mhocko@suse.com>

Ohh, an please add
Fixes: 1d82de618ddd ("mm, vmscan: make kswapd reclaim in terms of nodes")
-- 
Michal Hocko
SUSE Labs