linux-ext4 - Re: [PATCH] jbd2: avoid the concurrent data writeback

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Date:	Mon, 15 Nov 2010 13:54:20 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	"Tang, Feng" <feng.tang@...el.com>
Cc:	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] jbd2: avoid the concurrent data writeback

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Tang,

Good catch!

On Mon, Nov 15, 2010 at 09:25:33PM +0800, Tang, Feng wrote:
> When dd a big file to an ext4 partition, it is very likely to happen
> that both the background flush thread and kjounald try to do data
> writeback for it, and ext4_witepage may be called 100, 000 times by
> journal_submit_inode_data_buffers() without really writing one page
> back (skipped), as those pages haven't had disk blocks allocated yet.

The above changelog could show a bit more details (to help me
understand it :).

Does it happen frequently and hence has measurable overheads?

Is it safe to skip the inode? Another alternative is to wait for it:
with inode_wait_for_writeback(inode).

> This could be find by a simple test case with ftrace:
> $ sync;
> $ echo 40960 > buffer_size_kb;echo 1 > events/writeback/enable;echo 1 > events/jbd2/enable;echo 1 > events/ext4/enable;
> $ dd if=/dev/zero of=/home/test/1g.bin bs=1M count=1024;sync;
> $ cat trace > /home/test/jbd2_ext4_1g_dd.log
> 
> This patch will check if the inode is under data syncing, if yes then
> don't start the writeback from kjournald
> 
> Signed-off-by: Feng Tang <feng.tang@...el.com>
> ---
>  fs/jbd2/commit.c |    8 ++++++++
>  1 files changed, 8 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
> index f3ad159..8a1978d 100644
> --- a/fs/jbd2/commit.c
> +++ b/fs/jbd2/commit.c
> @@ -181,6 +181,14 @@ static int journal_submit_inode_data_buffers(struct address_space *mapping)
>  		.range_end = i_size_read(mapping->host),
>  	};
>  
> +	spin_lock(&inode_lock);
> +	/* If this inode is under data syncing, then just quit */

Comments should go above the whole code block. And the above comment
does not really say anything. Say something *why* code like this
rather than *what* the code is doing. 

> +	if (mapping->host->i_state & I_SYNC) {
> +		spin_unlock(&inode_lock);
> +		return 0;
> +	}
> +	spin_unlock(&inode_lock);
> +
>  	ret = generic_writepages(mapping, &wbc);
>  	return ret;
>  }

Thanks,
Fengguang
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