Re: [PATCH 3/5] writeback: fix dirtied pages accounting on sub-page writes

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

On Tue, Nov 22, 2011 at 08:11:27AM +0800, Jan Kara wrote:
> On Mon 21-11-11 21:03:45, Wu Fengguang wrote:
> > 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 nr_dirtied at page dirty time.
>   Well, but after this change, the interface balance_dirty_ratelimited_nr()
> is strange because the argument is only used for per-CPU ratelimiting and
> not for per-task ratelimiting...

Yeah I was vaguely aware of this... and still choose to ignore this
since the patchset looked already forbiddingly large at the time ;)

> So if you do this switch then I'd also
> switch bdp_ratelimits to get consistent results and a clean interface and
> completely kill balance_dirty_pages_ratelimited_nr().

Following your suggestions to change ratelimiting as well :)

I'll do the interface change with a standalone patch.

Thanks,
Fengguang
---
Subject: writeback: fix dirtied pages accounting on sub-page writes
Date: Thu Apr 14 07:52:37 CST 2011

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.

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	2011-11-22 16:59:48.000000000 +0800
+++ linux-next/mm/page-writeback.c	2011-11-22 17:12:20.000000000 +0800
@@ -1231,8 +1231,6 @@ 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
@@ -1243,12 +1241,9 @@ 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
@@ -1743,6 +1738,8 @@ void account_page_dirtied(struct page *p
 		__inc_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
 		task_dirty_inc(current);
 		task_io_account_write(PAGE_CACHE_SIZE);
+		current->nr_dirtied++;
+		__get_cpu_var(bdp_ratelimits)++;
 	}
 }
 EXPORT_SYMBOL(account_page_dirtied);