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Date:   Fri, 21 Jun 2019 15:16:33 +0100
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Alan Jenkins <alan.christopher.jenkins@gmail.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org
Subject: Re: [PATCH] mm: fix setting the high and low watermarks
Message-ID: <20190621141633.GA2978@techsingularity.net>
References: <20190621114325.711-1-alan.christopher.jenkins@gmail.com>
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On Fri, Jun 21, 2019 at 12:43:25PM +0100, Alan Jenkins wrote:
> When setting the low and high watermarks we use min_wmark_pages(zone).
> I guess this is to reduce the line length.  But we forgot that this macro
> includes zone->watermark_boost.  We need to reset zone->watermark_boost
> first.  Otherwise the watermarks will be set inconsistently.
> 
> E.g. this could cause inconsistent values if the watermarks have been
> boosted, and then you change a sysctl which triggers
> __setup_per_zone_wmarks().
> 
> I strongly suspect this explains why I have seen slightly high watermarks.
> Suspicious-looking zoneinfo below - notice high-low != low-min.
> 
> Node 0, zone   Normal
>   pages free     74597
>         min      9582
>         low      34505
>         high     36900
> 
> https://unix.stackexchange.com/questions/525674/my-low-and-high-watermarks-seem-higher-than-predicted-by-documentation-sysctl-vm/525687
> 
> Signed-off-by: Alan Jenkins <alan.christopher.jenkins@gmail.com>
> Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external
>                       fragmentation event occurs")
> Cc: stable@vger.kernel.org

Either way

Acked-by: Mel Gorman <mgorman@techsingularity.net>

-- 
Mel Gorman
SUSE Labs