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Re: [PATCH 1/5] mm: vmscan: Do not writeback filesystem pages in direct

To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Subject: Re: [PATCH 1/5] mm: vmscan: Do not writeback filesystem pages in direct reclaim
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Thu, 14 Jul 2011 00:46:43 -0400
Cc: Mel Gorman <mgorman@xxxxxxx>, Linux-MM <linux-mm@xxxxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>, XFS <xfs@xxxxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, Johannes Weiner <jweiner@xxxxxxxxxx>, Wu Fengguang <fengguang.wu@xxxxxxxxx>, Jan Kara <jack@xxxxxxx>, Rik van Riel <riel@xxxxxxxxxx>, Minchan Kim <minchan.kim@xxxxxxxxx>
In-reply-to: <20110714103801.83e10fdb.kamezawa.hiroyu@xxxxxxxxxxxxxx>
References: <1310567487-15367-1-git-send-email-mgorman@xxxxxxx> <1310567487-15367-2-git-send-email-mgorman@xxxxxxx> <20110714103801.83e10fdb.kamezawa.hiroyu@xxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Thu, Jul 14, 2011 at 10:38:01AM +0900, KAMEZAWA Hiroyuki wrote:
> > +                   /*
> > +                    * Only kswapd can writeback filesystem pages to
> > +                    * avoid risk of stack overflow
> > +                    */
> > +                   if (page_is_file_cache(page) && !current_is_kswapd()) {
> > +                           inc_zone_page_state(page, NR_VMSCAN_WRITE_SKIP);
> > +                           goto keep_locked;
> > +                   }
> > +
> 
> 
> This will cause tons of memcg OOM kill because we have no help of kswapd 
> (now).

XFS and btrfs already disable writeback from memcg context, as does ext4
for the typical non-overwrite workloads, and none has fallen apart.

In fact there's no way we can enable them as the memcg calling contexts
tend to have massive stack usage.

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