linux-kernel - Re: [PATCH 3/5] writeback: fix dirtied pages accounting on sub-page writes

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Date:	Tue, 22 Nov 2011 01:11:27 +0100
From:	Jan Kara <jack@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	linux-fsdevel@...r.kernel.org, Jan Kara <jack@...e.cz>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/5] writeback: fix dirtied pages accounting on
 sub-page writes

On Mon 21-11-11 21:03:45, 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.
> 
> Fix it by accounting nr_dirtied at page dirty time.
  Well, but after this change, the interface balance_dirty_ratelimited_nr()
is strange because the argument is only used for per-CPU ratelimiting and
not for per-task ratelimiting... So if you do this switch then I'd also
switch bdp_ratelimits to get consistent results and a clean interface and
completely kill balance_dirty_pages_ratelimited_nr().

								Honza

> 
> Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> ---
>  mm/page-writeback.c |    3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> --- linux-next.orig/mm/page-writeback.c	2011-11-17 20:57:13.000000000 +0800
> +++ linux-next/mm/page-writeback.c	2011-11-17 20:57:15.000000000 +0800
> @@ -1224,8 +1224,6 @@ 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
> @@ -1734,6 +1732,7 @@ 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++;
>  	}
>  }
>  EXPORT_SYMBOL(account_page_dirtied);
> 
> 
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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