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Subject[PATCH] writeback: fix BDI_WRITTEN accounting disturbing bdi->completions
Peter,

This is an amazing bug. I'm not sure how the accounting goes wrong in
some tricky way. But you can compare the exact bdi proportions pattern
before/after patch. The gray "bdi setpoint" lines are vastly different.

---
When increasing BDI_WRITTEN together with bdi->completions inside the
same local irq disabling block, bdi_thresh is found to go wild in the
1 disk + 1 usb stick writeback test case. Fix it by moving BDI_WRITTEN
accounting out.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
mm/page-writeback.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

--- linux.orig/mm/page-writeback.c 2011-09-01 21:43:45.312000020 +0800
+++ linux/mm/page-writeback.c 2011-09-01 21:46:10.656000006 +0800
@@ -230,7 +230,6 @@ int dirty_bytes_handler(struct ctl_table
*/
static inline void __bdi_writeout_inc(struct backing_dev_info *bdi)
{
- __inc_bdi_stat(bdi, BDI_WRITTEN);
__prop_inc_percpu_max(&vm_completions, &bdi->completions,
bdi->max_prop_frac);
}
@@ -239,6 +238,7 @@ void bdi_writeout_inc(struct backing_dev
{
unsigned long flags;

+ inc_bdi_stat(bdi, BDI_WRITTEN);
local_irq_save(flags);
__bdi_writeout_inc(bdi);
local_irq_restore(flags);
@@ -1577,6 +1577,7 @@ int test_clear_page_writeback(struct pag
PAGECACHE_TAG_WRITEBACK);
if (bdi_cap_account_writeback(bdi)) {
__dec_bdi_stat(bdi, BDI_WRITEBACK);
+ __inc_bdi_stat(bdi, BDI_WRITTEN);
__bdi_writeout_inc(bdi);
}
}[unhandled content-type:image/png][unhandled content-type:image/png]
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