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

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 1/5] mm: vmscan: Do not writeback filesystem pages in direct reclaim
From: Mel Gorman <mgorman@xxxxxxx>
Date: Thu, 14 Jul 2011 07:17:37 +0100
Cc: Linux-MM <linux-mm@xxxxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>, XFS <xfs@xxxxxxxxxxx>, 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: <20110713233449.GU23038@dastard>
References: <1310567487-15367-1-git-send-email-mgorman@xxxxxxx> <1310567487-15367-2-git-send-email-mgorman@xxxxxxx> <20110713233449.GU23038@dastard>
User-agent: Mutt/1.5.21 (2010-09-15)
On Thu, Jul 14, 2011 at 09:34:49AM +1000, Dave Chinner wrote:
> On Wed, Jul 13, 2011 at 03:31:23PM +0100, Mel Gorman wrote:
> > From: Mel Gorman <mel@xxxxxxxxx>
> > 
> > When kswapd is failing to keep zones above the min watermark, a process
> > will enter direct reclaim in the same manner kswapd does. If a dirty
> > page is encountered during the scan, this page is written to backing
> > storage using mapping->writepage.
> > 
> > This causes two problems. First, it can result in very deep call
> > stacks, particularly if the target storage or filesystem are complex.
> > Some filesystems ignore write requests from direct reclaim as a result.
> > The second is that a single-page flush is inefficient in terms of IO.
> > While there is an expectation that the elevator will merge requests,
> > this does not always happen. Quoting Christoph Hellwig;
> > 
> >     The elevator has a relatively small window it can operate on,
> >     and can never fix up a bad large scale writeback pattern.
> > 
> > This patch prevents direct reclaim writing back filesystem pages by
> > checking if current is kswapd. Anonymous pages are still written to
> > swap as there is not the equivalent of a flusher thread for anonymos
> > pages. If the dirty pages cannot be written back, they are placed
> > back on the LRU lists.
> > 
> > Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> 
> Ok, so that makes the .writepage checks in ext4, xfs and btrfs for this
> condition redundant. In effect the patch should be a no-op for those
> filesystems. Can you also remove the checks in the filesystems?
> 

I'll convert them to warnings just in case it regresses due to an
oversight.

-- 
Mel Gorman
SUSE Labs

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