linux-kernel - Re: [PATCH 2/4] writeback: avoid duplicate balance_dirty_pages

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Date:	Wed, 13 Apr 2011 23:53:07 +0200
From:	Jan Kara <jack@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 2/4] writeback: avoid duplicate
 balance_dirty_pages_ratelimited() calls

On Wed 13-04-11 16:59:39, Wu Fengguang wrote:
> When dd in 512bytes, balance_dirty_pages_ratelimited() could be called 8
> times for the same page, but obviously the page is only dirtied once.
> 
> Fix it with a (slightly racy) PageDirty() test.
> 
> Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> ---
>  mm/filemap.c |    5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> --- linux-next.orig/mm/filemap.c	2011-04-13 16:46:01.000000000 +0800
> +++ linux-next/mm/filemap.c	2011-04-13 16:47:26.000000000 +0800
> @@ -2313,6 +2313,7 @@ static ssize_t generic_perform_write(str
>  	long status = 0;
>  	ssize_t written = 0;
>  	unsigned int flags = 0;
> +	unsigned int dirty;
>  
>  	/*
>  	 * Copies from kernel address space cannot fail (NFSD is a big user).
> @@ -2361,6 +2362,7 @@ again:
>  		pagefault_enable();
>  		flush_dcache_page(page);
>  
> +		dirty = PageDirty(page);
  This isn't completely right as we sometimes dirty the page in
->write_begin() (see e.g. block_write_begin() when we allocate blocks under
an already uptodate page) and in such cases we would not call
balance_dirty_pages(). So I'm not sure we can really do this
optimization (although it's sad)...

>  		mark_page_accessed(page);
>  		status = a_ops->write_end(file, mapping, pos, bytes, copied,
>  						page, fsdata);
> @@ -2387,7 +2389,8 @@ again:
>  		pos += copied;
>  		written += copied;
>  
> -		balance_dirty_pages_ratelimited(mapping);
> +		if (!dirty)
> +			balance_dirty_pages_ratelimited(mapping);
>  
>  	} while (iov_iter_count(i));

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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