Re: [PATCH 1/8] mm: vmscan: Do not writeback filesystem pages in direct reclaim

Mel Gorman
2011-08-02T11:21:56Z

On Mon, Aug 01, 2011 at 12:06:06AM +0900, Minchan Kim wrote:
> On Thu, Jul 21, 2011 at 05:28:43PM +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>
>

Thanks

> Nitpick.
> We can change description of should_reclaim_stall.
>
> "Returns true if the caller should wait to clean dirty/writeback pages"
> ->
> "Returns true if direct reclaimer should wait to clean writeback pages"
>

Not a nitpick. At least one check for RECLAIM_MODE_SYNC is no longer
reachable. I've added a new patch that updates the comment and has
synchronous direct reclaim wait on pages under writeback.

--
Mel Gorman
SUSE Labs