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

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Message-ID: <20170203212411.GA12133@cmpxchg.org>
Date:   Fri, 3 Feb 2017 16:24:11 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Mel Gorman <mgorman@...e.de>
Cc:     'Andrew Morton' <akpm@...ux-foundation.org>,
        Hillf Danton <hillf.zj@...baba-inc.com>,
        'Michal Hocko' <mhocko@...e.com>,
        '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 Fri, Feb 03, 2017 at 08:32:22PM +0000, 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>

Acked-by: Johannes Weiner <hannes@...xchg.org>