linux-kernel - Re: [PATCH 2/5] writeback: dirty position control

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Date:	Mon, 8 Aug 2011 22:11:28 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Greg Thelen <gthelen@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Andrea Righi <arighi@...eler.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/5] writeback: dirty position control

On Mon, Aug 08, 2011 at 09:46:33PM +0800, Peter Zijlstra wrote:
> On Sat, 2011-08-06 at 16:44 +0800, Wu Fengguang wrote:
> > +static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
> > +                                       unsigned long thresh,
> > +                                       unsigned long dirty,
> > +                                       unsigned long bdi_thresh,
> > +                                       unsigned long bdi_dirty)
> > +{
> > +       unsigned long limit = hard_dirty_limit(thresh);
> > +       unsigned long origin;
> > +       unsigned long goal;
> > +       unsigned long long span;
> > +       unsigned long long pos_ratio;   /* for scaling up/down the rate limit */
> > +
> > +       if (unlikely(dirty >= limit))
> > +               return 0;
> > +
> > +       /*
> > +        * global setpoint
> > +        */
> > +       goal = thresh - thresh / DIRTY_SCOPE;
> > +       origin = 4 * thresh;
> > +
> > +       if (unlikely(origin < limit && dirty > (goal + origin) / 2)) {
> > +               origin = limit;                 /* auxiliary control line */
> > +               goal = (goal + origin) / 2;
> > +               pos_ratio >>= 1; 
> 
> use before init?

Yeah it's embarrassing, I find this bug all the way back to the initial version...

It's actually dead code because (origin < limit) should never happen.
I feel so good being able to drop 5 more lines of code :)

Thanks,
Fengguang
---

--- linux-next.orig/mm/page-writeback.c	2011-08-08 21:56:11.000000000 +0800
+++ linux-next/mm/page-writeback.c	2011-08-08 22:04:48.000000000 +0800
@@ -538,11 +538,6 @@ static unsigned long bdi_position_ratio(
 	goal = thresh - thresh / DIRTY_SCOPE;
 	origin = 4 * thresh;
 
-	if (unlikely(origin < limit && dirty > (goal + origin) / 2)) {
-		origin = limit;			/* auxiliary control line */
-		goal = (goal + origin) / 2;
-		pos_ratio >>= 1;
-	}
 	pos_ratio = origin - dirty;
 	pos_ratio <<= BANDWIDTH_CALC_SHIFT;
 	do_div(pos_ratio, origin - goal + 1);
--
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