linux-kernel - Re: [PATCH] writeback: prevent bandwidth calculation overflow

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Date:	Fri, 19 Nov 2010 00:13:56 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, "Li, Shaohua" <shaohua.li@...el.com>,
	Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Theodore Ts'o <tytso@....edu>,
	Chris Mason <chris.mason@...cle.com>,
	Mel Gorman <mel@....ul.ie>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] writeback: prevent bandwidth calculation overflow

On Fri, Nov 19, 2010 at 12:02:01AM +0800, Peter Zijlstra wrote:
> On Thu, 2010-11-18 at 23:44 +0800, Wu Fengguang wrote:
> > +               pause = HZ * pages_dirtied / (bw + 1);
> 
> Shouldn't that be using something like div64_u64 ?

Thanks for review. Here is the updated patch using div64_u64().

---
Subject: writeback: prevent bandwidth calculation overflow
Date: Thu Nov 18 12:55:42 CST 2010

On 32bit kernel, bdi->write_bandwidth can express at most 4GB/s.

However the current calculation code can overflow when disk bandwidth
reaches 800MB/s.  Fix it by using "long long" and div64_u64() in the
calculations.

And further change its unit from bytes/second to pages/second.
That allows up to 16TB/s bandwidth in 32bit kernel.

CC: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Acked-by: Rik van Riel <riel@...hat.com>
Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
---
 mm/backing-dev.c    |    4 ++--
 mm/page-writeback.c |   11 +++++------
 2 files changed, 7 insertions(+), 8 deletions(-)

--- linux-next.orig/mm/page-writeback.c	2010-11-18 12:42:58.000000000 +0800
+++ linux-next/mm/page-writeback.c	2010-11-19 00:08:23.000000000 +0800
@@ -494,7 +494,7 @@ void bdi_update_write_bandwidth(struct b
 	unsigned long written;
 	unsigned long elapsed;
 	unsigned long bw;
-	unsigned long w;
+	unsigned long long w;
 
 	if (*bw_written == 0)
 		goto snapshot;
@@ -513,7 +513,7 @@ void bdi_update_write_bandwidth(struct b
 		goto snapshot;
 
 	written = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]) - *bw_written;
-	bw = (HZ * PAGE_CACHE_SIZE * written + elapsed/2) / elapsed;
+	bw = (HZ * written + elapsed/2) / elapsed;
 	w = min(elapsed / unit_time, 128UL);
 	bdi->write_bandwidth = (bdi->write_bandwidth * (1024-w) + bw * w) >> 10;
 	bdi->write_bandwidth_update_time = jiffies;
@@ -539,7 +539,7 @@ static void balance_dirty_pages(struct a
 	unsigned long dirty_thresh;
 	unsigned long bdi_thresh;
 	unsigned long task_thresh;
-	unsigned long bw;
+	unsigned long long bw;
 	unsigned long pause = 0;
 	bool dirty_exceeded = false;
 	struct backing_dev_info *bdi = mapping->backing_dev_info;
@@ -602,8 +602,7 @@ static void balance_dirty_pages(struct a
 		 * of dirty pages have been cleaned during our pause time.
 		 */
 		if (nr_dirty < dirty_thresh &&
-		    bdi_prev_dirty - bdi_dirty >
-		    bdi->write_bandwidth >> (PAGE_CACHE_SHIFT + 2))
+		    bdi_prev_dirty - bdi_dirty > bdi->write_bandwidth / 4)
 			break;
 		bdi_prev_dirty = bdi_dirty;
 
@@ -626,7 +625,7 @@ static void balance_dirty_pages(struct a
 		bw = bw * (task_thresh - bdi_dirty);
 		bw = bw / (bdi_thresh / TASK_SOFT_DIRTY_LIMIT + 1);
 
-		pause = HZ * (pages_dirtied << PAGE_CACHE_SHIFT) / (bw + 1);
+		pause = div64_u64(HZ * pages_dirtied, bw + 1);
 		pause = clamp_val(pause, 1, HZ/10);
 
 pause:
--- linux-next.orig/mm/backing-dev.c	2010-11-18 14:24:45.000000000 +0800
+++ linux-next/mm/backing-dev.c	2010-11-18 14:27:00.000000000 +0800
@@ -103,7 +103,7 @@ static int bdi_debug_stats_show(struct s
 		   (unsigned long) K(bdi_stat(bdi, BDI_RECLAIMABLE)),
 		   K(bdi_thresh), K(dirty_thresh), K(background_thresh),
 		   (unsigned long) K(bdi_stat(bdi, BDI_WRITTEN)),
-		   (unsigned long) bdi->write_bandwidth >> 10,
+		   (unsigned long) K(bdi->write_bandwidth),
 		   nr_dirty, nr_io, nr_more_io,
 		   !list_empty(&bdi->bdi_list), bdi->state);
 #undef K
@@ -662,7 +662,7 @@ int bdi_init(struct backing_dev_info *bd
 			goto err;
 	}
 
-	bdi->write_bandwidth = 100 << 20;
+	bdi->write_bandwidth = (100 << 20) / PAGE_CACHE_SIZE;
 	bdi->dirty_exceeded = 0;
 	err = prop_local_init_percpu(&bdi->completions);
 
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