linux-kernel - Re: [PATCH 2/2] ocfs2: No need to zero pages past i_size. i

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Date:	Sun, 04 Jul 2010 23:13:01 +0800
From:	Tao Ma <tao.ma@...cle.com>
To:	Dave Chinner <david@...morbit.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	ocfs2-devel@....oracle.com, Dave Chinner <dchinner@...hat.com>,
	Christoph Hellwig <hch@....de>, Mark Fasheh <mfasheh@...e.com>,
	Joel Becker <joel.becker@...cle.com>
Subject: Re: [PATCH 2/2] ocfs2: No need to zero pages past i_size. i_size
 v2

Hi Joel,

On 07/04/2010 05:33 AM, Joel Becker wrote:
> 	Here's the second patch, the one that keeps us from zeroing
> pages past i_size.  This should keep ocfs2 and Dave's writeback patch
> happy.
>
> Joel
>
> -------------------------------------------------------
>
> When ocfs2 fills a hole, it does so by allocating clusters.  When a
> cluster is larger than the write, ocfs2 must zero the portions of the
> cluster outside of the write.  If the clustersize is smaller than a
> pagecache page, this is handled by the normal pagecache mechanisms, but
> when the clustersize is larger than a page, ocfs2's write code will zero
> the pages adjacent to the write.  This makes sure the entire cluster is
> zeroed correctly.
>
> Currently ocfs2 behaves exactly the same when writing past i_size.
> However, this means ocfs2 is writing zeroed pages for portions of a new
> cluster that are beyond i_size.  The page writeback code isn't expecting
> this.  It treats all pages past the one containing i_size as left behind
> due to a previous truncate operation.
>
> Thankfully, ocfs2 calculates the number of pages it will be working on
> up front.  The rest of the write code merely honors the original
> calculation.  We can simply trim the number of pages to only cover the
> actual file data.
>
> Signed-off-by: Joel Becker<joel.becker@...cle.com>
> ---
>   fs/ocfs2/aops.c |   15 +++++++++++----
>   1 files changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/fs/ocfs2/aops.c b/fs/ocfs2/aops.c
> index 96e6aeb..e90ad74 100644
> --- a/fs/ocfs2/aops.c
> +++ b/fs/ocfs2/aops.c
<snip>
> @@ -1142,11 +1143,17 @@ static int ocfs2_grab_pages_for_write(struct address_space *mapping,
>   	/*
>   	 * Figure out how many pages we'll be manipulating here. For
>   	 * non allocating write, we just change the one
> -	 * page. Otherwise, we'll need a whole clusters worth.
> +	 * page. Otherwise, we'll need a whole clusters worth.  If we're
> +	 * writing past i_size, we only need enough pages to cover the
> +	 * last page of the write.
The comments for the whole function before the function name also needs 
this change accordingly?
>   	 */
>   	if (new) {
>   		wc->w_num_pages = ocfs2_pages_per_cluster(inode->i_sb);
>   		start = ocfs2_align_clusters_to_page_index(inode->i_sb, cpos);
> +		/* This is the index *past* the write */
> +		end_index = ((user_pos + user_len)>>  PAGE_CACHE_SHIFT) + 1;
should it be
		end_index = ((user_pos + user_len - 1) >> PAGE_CACHE_SHIFT) + 1?


> +		if ((start + wc->w_num_pages)>  end_index)
> +			wc->w_num_pages = end_index - start;
I just noticed that the below loop in ocfs2_grab_pages_for_write is
for (i = 0; i < wc->w_num_pages; i++)

I guess w_num_pages should be set to end_index - 
start_page_of_the_cluster so that we can make sure we grab all the pages 
in this cluster until i_size?

Regards,
Tao
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