Re: [PATCH 1/7] mm: vmscan: Do not writeback filesystem pages in direct reclaim

[Johannes Weiner <jweiner@redhat.com>]

On Wed, Aug 10, 2011 at 11:47:14AM +0100, Mel Gorman wrote:
> From: Mel Gorman <mel@csn.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 anonymous
> pages. If the dirty pages cannot be written back, they are placed
> back on the LRU lists. There is now a direct dependency on dirty page
> balancing to prevent too many pages in the system being dirtied which
> would prevent reclaim making forward progress.
> 
> Signed-off-by: Mel Gorman <mgorman@suse.de>
> Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

Acked-by: Johannes Weiner <jweiner@redhat.com>