[PATCH] balance_dirty_pages - exit loop when no more pages available

[richard kennedy <richard@rsk.demon.co.uk>]

exit loop in balance_dirty_pages when no more pages available to write

On a bdi that has very little traffic balance_dirty_pages can loop
needlessly waiting until do_writepages has written enough pages.

do_writepages will return encountered_congestion==0 && nr_to_write > 0 
when it has completed a pass but did not find enough pages available to
write. balance_dirty_pages ignores this and keeps looping until a total
of chunk pages was written. 

this patch adds an extra exit condition to break out of the loop in this
case. 

I've tested this on my amd64 desktop, and I also have a version of this
patch that includes a printk and a test case that occasionally does
trigger this condition.  

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>

------
--- linux-2.6.22.1/mm/page-writeback.c.orig	2007-07-30 16:36:09.000000000 +0100
+++ linux-2.6.22.1/mm/page-writeback.c	2007-07-31 16:26:43.000000000 +0100
@@ -250,6 +250,8 @@ static void balance_dirty_pages(struct a
 			pages_written += write_chunk - wbc.nr_to_write;
 			if (pages_written >= write_chunk)
 				break;		/* We've done our duty */
+			if (!wbc.encountered_congestion && wbc.nr_to_write > 0)
+				break;	/* didn't find enough to do */
 		}
 		congestion_wait(WRITE, HZ/10);
 	}


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