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Subject[PATCH] writeback: prevent bandwidth calculation overflow
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 swapping the order of
multiplication/division. And further, change its unit to pages/second
rather than bytes/second. That allows up to 16TB/s bandwidth in 32bit
kernel.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
mm/backing-dev.c | 4 ++--
mm/page-writeback.c | 13 ++++++-------
2 files changed, 8 insertions(+), 9 deletions(-)

--- linux-next.orig/mm/page-writeback.c 2010-11-18 12:42:58.000000000 +0800
+++ linux-next/mm/page-writeback.c 2010-11-18 14:30:10.000000000 +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;
@@ -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;

@@ -620,13 +619,13 @@ static void balance_dirty_pages(struct a
* time when there are lots of dirtiers.
*/
bw = bdi->write_bandwidth;
- bw = bw * (bdi_thresh - bdi_dirty);
bw = bw / (bdi_thresh / BDI_SOFT_DIRTY_LIMIT + 1);
+ bw = bw * (bdi_thresh - bdi_dirty);

- bw = bw * (task_thresh - bdi_dirty);
bw = bw / (bdi_thresh / TASK_SOFT_DIRTY_LIMIT + 1);
+ bw = bw * (task_thresh - bdi_dirty);

- pause = HZ * (pages_dirtied << PAGE_CACHE_SHIFT) / (bw + 1);
+ pause = 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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