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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@xxxxxxxxx>
> ---
> 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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