linux-kernel - Re: [PATCH 2/3] mm, vmscan: consider eligible zones in get_scan

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Message-ID: <20170118164647.GB32495@cmpxchg.org>
Date:   Wed, 18 Jan 2017 11:46:47 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>, Minchan Kim <minchan@...nel.org>,
        Hillf Danton <hillf.zj@...baba-inc.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 2/3] mm, vmscan: consider eligible zones in get_scan_count

On Tue, Jan 17, 2017 at 11:37:01AM +0100, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> get_scan_count considers the whole node LRU size when
> - doing SCAN_FILE due to many page cache inactive pages
> - calculating the number of pages to scan
> 
> in both cases this might lead to unexpected behavior especially on 32b
> systems where we can expect lowmem memory pressure very often.
> 
> A large highmem zone can easily distort SCAN_FILE heuristic because
> there might be only few file pages from the eligible zones on the node
> lru and we would still enforce file lru scanning which can lead to
> trashing while we could still scan anonymous pages.
> 
> The later use of lruvec_lru_size can be problematic as well. Especially
> when there are not many pages from the eligible zones. We would have to
> skip over many pages to find anything to reclaim but shrink_node_memcg
> would only reduce the remaining number to scan by SWAP_CLUSTER_MAX
> at maximum. Therefore we can end up going over a large LRU many times
> without actually having chance to reclaim much if anything at all. The
> closer we are out of memory on lowmem zone the worse the problem will
> be.
> 
> Fix this by filtering out all the ineligible zones when calculating the
> lru size for both paths and consider only sc->reclaim_idx zones.
> 
> Acked-by: Minchan Kim <minchan@...nel.org>
> Acked-by: Hillf Danton <hillf.zj@...baba-inc.com>
> Signed-off-by: Michal Hocko <mhocko@...e.com>

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