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SubjectRe: [PATCH 3/7] writeback: fix dirtied pages accounting on sub-page writes
On Mon 28-11-11 21:53:41, Wu Fengguang wrote:
> When dd in 512bytes, generic_perform_write() calls
> balance_dirty_pages_ratelimited() 8 times for the same page, but
> obviously the page is only dirtied once.
Actually, for ppc where pages can be 64 KB the problem is even worse.

> Fix it by accounting tsk->nr_dirtied and bdp_ratelimits at page dirty time.
I was wondering about one more thing - couldn't we rather check in
generic_perform_write() whether the page was dirty before calling
->write_end and call balance_dirty_pages_ratelimited() only if it wasn't?

For generic_perform_write() it doesn't really matter that much since we
do things page-by-page anyway but other callers could be more efficient...

Honza

> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
> mm/page-writeback.c | 13 +++++--------
> 1 file changed, 5 insertions(+), 8 deletions(-)
>
> --- linux-next.orig/mm/page-writeback.c 2011-11-28 21:23:20.000000000 +0800
> +++ linux-next/mm/page-writeback.c 2011-11-28 21:23:23.000000000 +0800
> @@ -1239,8 +1239,6 @@ void balance_dirty_pages_ratelimited_nr(
> if (bdi->dirty_exceeded)
> ratelimit = min(ratelimit, 32 >> (PAGE_SHIFT - 10));
>
> - current->nr_dirtied += nr_pages_dirtied;
> -
> preempt_disable();
> /*
> * This prevents one CPU to accumulate too many dirtied pages without
> @@ -1251,12 +1249,9 @@ void balance_dirty_pages_ratelimited_nr(
> p = &__get_cpu_var(bdp_ratelimits);
> if (unlikely(current->nr_dirtied >= ratelimit))
> *p = 0;
> - else {
> - *p += nr_pages_dirtied;
> - if (unlikely(*p >= ratelimit_pages)) {
> - *p = 0;
> - ratelimit = 0;
> - }
> + else if (unlikely(*p >= ratelimit_pages)) {
> + *p = 0;
> + ratelimit = 0;
> }
> /*
> * Pick up the dirtied pages by the exited tasks. This avoids lots of
> @@ -1749,6 +1744,8 @@ void account_page_dirtied(struct page *p
> __inc_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE);
> __inc_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
> task_io_account_write(PAGE_CACHE_SIZE);
> + current->nr_dirtied++;
> + this_cpu_inc(bdp_ratelimits);
> }
> }
> EXPORT_SYMBOL(account_page_dirtied);
>
>
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR


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