[RFC][PATCH 7/7] writeback: balance_dirty_pages() shall write more than dirtied pages

[Wu Fengguang]

Some filesystem may choose to write much more than ratelimit_pages
before calling balance_dirty_pages_ratelimited_nr(). So it is safer to
determine number to write based on real number of dirtied pages.

The increased write_chunk may make the dirtier more bumpy.  This is
filesystem writers' duty not to dirty too much at a time without
checking the ratelimit.

Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 mm/page-writeback.c |   13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

--- linux.orig/mm/page-writeback.c	2009-09-09 21:19:21.000000000 +0800
+++ linux/mm/page-writeback.c	2009-09-09 21:25:45.000000000 +0800
@@ -44,12 +44,12 @@ static long ratelimit_pages = 32;
 /*
  * When balance_dirty_pages decides that the caller needs to perform some
  * non-background writeback, this is how many pages it will attempt to write.
- * It should be somewhat larger than RATELIMIT_PAGES to ensure that reasonably
+ * It should be somewhat larger than dirtied pages to ensure that reasonably
  * large amounts of I/O are submitted.
  */
-static inline long sync_writeback_pages(void)
+static inline long sync_writeback_pages(unsigned long dirtied)
 {
-	return ratelimit_pages + ratelimit_pages / 2;
+	return dirtied + dirtied / 2;
 }
 
 /* The following parameters are exported via /proc/sys/vm */
@@ -476,7 +476,8 @@ get_dirty_limits(unsigned long *pbackgro
  * If we're over `background_thresh' then pdflush is woken to perform some
  * writeout.
  */
-static void balance_dirty_pages(struct address_space *mapping)
+static void balance_dirty_pages(struct address_space *mapping,
+				unsigned long write_chunk)
 {
 	long nr_reclaimable, bdi_nr_reclaimable;
 	long nr_writeback, bdi_nr_writeback;
@@ -484,7 +485,6 @@ static void balance_dirty_pages(struct a
 	unsigned long dirty_thresh;
 	unsigned long bdi_thresh;
 	unsigned long pages_written = 0;
-	unsigned long write_chunk = sync_writeback_pages();
 
 	struct backing_dev_info *bdi = mapping->backing_dev_info;
 
@@ -638,9 +638,10 @@ void balance_dirty_pages_ratelimited_nr(
 	p =  &__get_cpu_var(bdp_ratelimits);
 	*p += nr_pages_dirtied;
 	if (unlikely(*p >= ratelimit)) {
+		ratelimit = sync_writeback_pages(*p);
 		*p = 0;
 		preempt_enable();
-		balance_dirty_pages(mapping);
+		balance_dirty_pages(mapping, ratelimit);
 		return;
 	}
 	preempt_enable();

-- 

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