Re: [PATCH 4/7] writeback: fix dirtied pages accounting on redirty

[Jan Kara <jack@suse.cz>]

On Mon 28-11-11 21:53:42, Wu Fengguang wrote:
> De-account the accumulative dirty counters on page redirty.
> 
> Page redirties (very common in ext4) will introduce mismatch between
> counters (a) and (b)
> 
> a) NR_DIRTIED, BDI_DIRTIED, tsk->nr_dirtied
> b) NR_WRITTEN, BDI_WRITTEN
> 
> This will introduce systematic errors in balanced_rate and result in
> dirty page position errors (ie. the dirty pages are no longer balanced
> around the global/bdi setpoints).
> 
> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
  You can add:

  Acked-by: Jan Kara <jack@suse.cz>

								Honza
> ---
>  include/linux/writeback.h |    2 ++
>  mm/page-writeback.c       |   19 +++++++++++++++++++
>  2 files changed, 21 insertions(+)
> 
> --- linux-next.orig/mm/page-writeback.c	2011-11-28 21:23:23.000000000 +0800
> +++ linux-next/mm/page-writeback.c	2011-11-28 21:23:24.000000000 +0800
> @@ -1806,6 +1806,24 @@ int __set_page_dirty_nobuffers(struct pa
>  EXPORT_SYMBOL(__set_page_dirty_nobuffers);
>  
>  /*
> + * Call this whenever redirtying a page, to de-account the dirty counters
> + * (NR_DIRTIED, BDI_DIRTIED, tsk->nr_dirtied), so that they match the written
> + * counters (NR_WRITTEN, BDI_WRITTEN) in long term. The mismatches will lead to
> + * systematic errors in balanced_dirty_ratelimit and the dirty pages position
> + * control.
> + */
> +void account_page_redirty(struct page *page)
> +{
> +	struct address_space *mapping = page->mapping;
> +	if (mapping && mapping_cap_account_dirty(mapping)) {
> +		current->nr_dirtied--;
> +		dec_zone_page_state(page, NR_DIRTIED);
> +		dec_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
> +	}
> +}
> +EXPORT_SYMBOL(account_page_redirty);
> +
> +/*
>   * When a writepage implementation decides that it doesn't want to write this
>   * page for some reason, it should redirty the locked page via
>   * redirty_page_for_writepage() and it should then unlock the page and return 0
> @@ -1813,6 +1831,7 @@ EXPORT_SYMBOL(__set_page_dirty_nobuffers
>  int redirty_page_for_writepage(struct writeback_control *wbc, struct page *page)
>  {
>  	wbc->pages_skipped++;
> +	account_page_redirty(page);
>  	return __set_page_dirty_nobuffers(page);
>  }
>  EXPORT_SYMBOL(redirty_page_for_writepage);
> --- linux-next.orig/include/linux/writeback.h	2011-11-28 21:23:20.000000000 +0800
> +++ linux-next/include/linux/writeback.h	2011-11-28 21:23:24.000000000 +0800
> @@ -197,6 +197,8 @@ void writeback_set_ratelimit(void);
>  void tag_pages_for_writeback(struct address_space *mapping,
>  			     pgoff_t start, pgoff_t end);
>  
> +void account_page_redirty(struct page *page);
> +
>  /* pdflush.c */
>  extern int nr_pdflush_threads;	/* Global so it can be exported to sysctl
>  				   read-only. */
> 
> 
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR