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SubjectRe: [PATCH] writeback: prevent bandwidth calculation overflow
On Sat, Nov 20, 2010 at 12:06:53AM +0800, Michal Hocko wrote:
> On Fri 19-11-10 00:13:56, Wu Fengguang wrote:
> > 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@chello.nl>
> > Acked-by: Rik van Riel <riel@redhat.com>
> > Signed-off-by: Wu Fengguang <fengguang.wu@intel.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;
>
> Sorry for a dumb question, but where did PAGE_CACHE_SIZE part go?

Because write_bandwidth's unit is bumped from bytes/s to pages/s,
so that it can express much higher bandwidth.

I've updated the patch again, and will submit more fixes tomorrow.

Thanks,
Fengguang
---
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@chello.nl>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
include/linux/backing-dev.h | 5 +++--
mm/backing-dev.c | 4 ++--
mm/page-writeback.c | 14 +++++++-------
3 files changed, 12 insertions(+), 11 deletions(-)

--- linux-next.orig/mm/page-writeback.c 2010-11-19 09:56:14.000000000 +0800
+++ linux-next/mm/page-writeback.c 2010-11-19 22:18:57.000000000 +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;
@@ -513,9 +513,10 @@ 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 = (bdi->write_bandwidth * (1024-w) +
+ bw * w + 1023) >> 10;
bdi->write_bandwidth_update_time = jiffies;
snapshot:
*bw_written = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]);
@@ -539,7 +540,7 @@ 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 +603,7 @@ 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 > (long)bdi->write_bandwidth / 4)
break;
bdi_prev_dirty = bdi_dirty;

@@ -626,7 +626,7 @@ 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 = HZ * pages_dirtied / ((unsigned long)bw + 1);
pause = clamp_val(pause, 1, HZ/10);

pause:
--- linux-next.orig/mm/backing-dev.c 2010-11-19 09:56:14.000000000 +0800
+++ linux-next/mm/backing-dev.c 2010-11-19 22:18:19.000000000 +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);

--- linux-next.orig/include/linux/backing-dev.h 2010-11-19 19:43:05.000000000 +0800
+++ linux-next/include/linux/backing-dev.h 2010-11-19 22:18:19.000000000 +0800
@@ -74,9 +74,10 @@ struct backing_dev_info {

struct percpu_counter bdi_stat[NR_BDI_STAT_ITEMS];

- struct prop_local_percpu completions;
+ unsigned long write_bandwidth;
unsigned long write_bandwidth_update_time;
- int write_bandwidth;
+
+ struct prop_local_percpu completions;
int dirty_exceeded;

unsigned int min_ratio;

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