[PATCH 3/7] writeback: fix dirtied pages accounting on sub-page writes

[Wu Fengguang <fengguang.wu@intel.com>]

[-- Attachment #1: writeback-accurate-task-dirtied.patch --]
[-- Type: text/plain, Size: 1640 bytes --]

When dd in 512bytes, generic_perform_write() calls
balance_dirty_pages_ratelimited() 8 times for the same page, but
obviously the page is only dirtied once.

Fix it by accounting tsk->nr_dirtied and bdp_ratelimits at page dirty time.

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 mm/page-writeback.c |   13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

--- linux-next.orig/mm/page-writeback.c	2011-11-28 21:23:20.000000000 +0800
+++ linux-next/mm/page-writeback.c	2011-11-28 21:23:23.000000000 +0800
@@ -1239,8 +1239,6 @@ void balance_dirty_pages_ratelimited_nr(
 	if (bdi->dirty_exceeded)
 		ratelimit = min(ratelimit, 32 >> (PAGE_SHIFT - 10));
 
-	current->nr_dirtied += nr_pages_dirtied;
-
 	preempt_disable();
 	/*
 	 * This prevents one CPU to accumulate too many dirtied pages without
@@ -1251,12 +1249,9 @@ void balance_dirty_pages_ratelimited_nr(
 	p =  &__get_cpu_var(bdp_ratelimits);
 	if (unlikely(current->nr_dirtied >= ratelimit))
 		*p = 0;
-	else {
-		*p += nr_pages_dirtied;
-		if (unlikely(*p >= ratelimit_pages)) {
-			*p = 0;
-			ratelimit = 0;
-		}
+	else if (unlikely(*p >= ratelimit_pages)) {
+		*p = 0;
+		ratelimit = 0;
 	}
 	/*
 	 * Pick up the dirtied pages by the exited tasks. This avoids lots of
@@ -1749,6 +1744,8 @@ void account_page_dirtied(struct page *p
 		__inc_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE);
 		__inc_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
 		task_io_account_write(PAGE_CACHE_SIZE);
+		current->nr_dirtied++;
+		this_cpu_inc(bdp_ratelimits);
 	}
 }
 EXPORT_SYMBOL(account_page_dirtied);