On Thu 01-09-11 22:40:41, Wu Fengguang wrote:
> 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.
I don't understand this - the patch is just NOP. The change in
test_clear_page_writeback() does absolutely nothing and the change in
bdi_writeout_inc() just changes:
local_irq_save(flags);
__inc_bdi_stat(bdi);
__prop_inc_percpu_max(&vm_completions, &bdi->completions,
bdi->max_prop_frac);
local_irq_restore(flags);
to:
local_irq_save(flags);
__inc_bdi_stat(bdi);
local_irq_restore(flags);
local_irq_save(flags);
__prop_inc_percpu_max(&vm_completions, &bdi->completions,
bdi->max_prop_frac);
local_irq_restore(flags);
So the difference must be in something else...
Honza
>
> Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> ---
> 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);
> }
> }
--
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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