linux-kernel - Re: [PATCH] mm, vmscan: Clear PGDAT

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Message-ID: <20170205101316.GC22713@dhcp22.suse.cz>
Date:   Sun, 5 Feb 2017 11:13:16 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Mel Gorman <mgorman@...e.de>
Cc:     'Andrew Morton' <akpm@...ux-foundation.org>,
        Hillf Danton <hillf.zj@...baba-inc.com>,
        'Johannes Weiner' <hannes@...xchg.org>,
        'Minchan Kim' <minchan.kim@...il.com>,
        'Rik van Riel' <riel@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm, vmscan: Clear PGDAT_WRITEBACK when zone is balanced

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@...e.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@...e.com>

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