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Subject[PATCH] writeback: remove the internal 5% low bound on dirty_ratio
The dirty_ratio was siliently limited in global_dirty_limits() to >= 5%.
This is not a user expected behavior. And it's inconsistent with
calc_period_shift(), which uses the plain vm_dirty_ratio value.

Let's rip the internal bound.

At the same time, fix balance_dirty_pages() to work with the
dirty_thresh=0 case. This allows applications to proceed when
dirty+writeback pages are all cleaned.

And ">" fits with the name "exceeded" better than ">=" does. Neil
think it is an aesthetic improvement as well as a functional one :)

CC: Jan Kara <jack@suse.cz>
CC: Rik van Riel <riel@redhat.com>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
Proposed-by: Con Kolivas <kernel@kolivas.org>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
mm/page-writeback.c | 14 ++++----------
1 file changed, 4 insertions(+), 10 deletions(-)

--- linux-next.orig/mm/page-writeback.c 2010-08-26 08:37:31.000000000 +0800
+++ linux-next/mm/page-writeback.c 2010-08-26 08:37:55.000000000 +0800
@@ -415,14 +415,8 @@ void global_dirty_limits(unsigned long *

if (vm_dirty_bytes)
dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
- else {
- int dirty_ratio;
-
- dirty_ratio = vm_dirty_ratio;
- if (dirty_ratio < 5)
- dirty_ratio = 5;
- dirty = (dirty_ratio * available_memory) / 100;
- }
+ else
+ dirty = (vm_dirty_ratio * available_memory) / 100;

if (dirty_background_bytes)
background = DIV_ROUND_UP(dirty_background_bytes, PAGE_SIZE);
@@ -542,8 +536,8 @@ static void balance_dirty_pages(struct a
* the last resort safeguard.
*/
dirty_exceeded =
- (bdi_nr_reclaimable + bdi_nr_writeback >= bdi_thresh)
- || (nr_reclaimable + nr_writeback >= dirty_thresh);
+ (bdi_nr_reclaimable + bdi_nr_writeback > bdi_thresh)
+ || (nr_reclaimable + nr_writeback > dirty_thresh);

if (!dirty_exceeded)
break;

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