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

Dave Chinner
2011-07-13T23:35:00Z

On Wed, Jul 13, 2011 at 03:31:23PM +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 anonymos
> pages. If the dirty pages cannot be written back, they are placed
> back on the LRU lists.
>
> Signed-off-by: Mel Gorman <mgorman@suse.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@fromorbit.com