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);
 			}
 		}