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

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Date:	Thu, 14 Jul 2011 09:34:49 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
	XFS <xfs@....sgi.com>, Christoph Hellwig <hch@...radead.org>,
	Johannes Weiner <jweiner@...hat.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Rik van Riel <riel@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>
Subject: Re: [PATCH 1/5] mm: vmscan: Do not writeback filesystem pages in
 direct reclaim

On Wed, Jul 13, 2011 at 03:31:23PM +0100, Mel Gorman wrote:
> From: Mel Gorman <mel@....ul.ie>
> 
> 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@...e.de>

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?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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