RE: [f2fs-dev] [PATCH 1/2] f2fs: avoid multiple node page writes due to inline_data

[Chao Yu <chao2.yu@samsung.com>]

Hi Jaegeuk,

> -----Original Message-----
> From: Jaegeuk Kim [mailto:jaegeuk@kernel.org]
> Sent: Sunday, January 24, 2016 4:16 AM
> To: linux-kernel@vger.kernel.org; linux-fsdevel@vger.kernel.org;
> linux-f2fs-devel@lists.sourceforge.net
> Cc: Jaegeuk Kim
> Subject: [f2fs-dev] [PATCH 1/2] f2fs: avoid multiple node page writes due to inline_data
> 
> The sceanrio is:
> 1. create fully node blocks
> 2. flush node blocks
> 3. write inline_data for all the node blocks again
> 4. flush node blocks redundantly
> 
> Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
> ---
>  fs/f2fs/data.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index 8d0d9ec..011456e 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -1622,14 +1622,22 @@ static int f2fs_write_end(struct file *file,
> 
>  	trace_f2fs_write_end(inode, pos, len, copied);
> 
> -	set_page_dirty(page);
> -
>  	if (pos + copied > i_size_read(inode)) {
>  		i_size_write(inode, pos + copied);
>  		mark_inode_dirty(inode);
> -		update_inode_page(inode);
>  	}
> 
> +	if (f2fs_has_inline_data(inode) &&
> +			is_inode_flag_set(F2FS_I(inode), FI_DATA_EXIST)) {
> +		int err = f2fs_write_inline_data(inode, page);

Oh, I'm sure this can fix that issue, but IMO:
a) this implementation has side-effect, it triggers inline data copying
between data page and node page whenever user write inline datas, so if
user updates inline data frequently, write-through approach would cause
memory copy overhead.
b) inline storm should be a rare case, as we didn't get any report about
problem for long time until Dave's, and write_end is a hot path, I think
it's better to be cautious to change our inline data cache policy for
fixing a rare issue in hot path.

What about delaying the merge operation? like:
1) as I proposed before, merging inline page into inode page when
detecting free_sections <= (node_secs + 2 * dent_secs + inline_secs).
2) merge inline page into inode page before writeback inode page in
sync_node_pages.

Thanks,

> +		if (err)
> +			set_page_dirty(page);
> +	} else {
> +		set_page_dirty(page);
> +	}
> +
> +	f2fs_write_inode(inode, NULL);
> +
>  	f2fs_put_page(page, 1);
>  	f2fs_update_time(F2FS_I_SB(inode), REQ_TIME);
>  	return copied;
> --
> 2.6.3
> 
> 
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