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:

quoted

From: Mel Gorman <redacted>

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 <redacted>

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

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