Re: [PATCH] ext4: don't order data when zeroing unwritten or delayed block

[Jan Kara]

On Mon 22-12-25 09:31:36, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@xxxxxxxxxx>
> 
> When zeroing out a written partial block, it is necessary to order the
> data to prevent exposing stale data on disk. However, if the buffer is
> unwritten or delayed, it is not allocated as written, so ordering the
> data is not required. This can prevent strange and unnecessary ordered
> writes when appending data across a region within a block.
> 
> Assume we have a 2K unwritten file on a filesystem with 4K blocksize,
> and buffered write from 3K to 4K. Before this patch,
> __ext4_block_zero_page_range() would add the range [2k,3k) to the
> ordered range, and then the JBD2 commit process would write back this
> block. However, it does nothing since the block is not mapped, this
							^^^ by this you
mean that the block is unwritten, don't you?

> folio will be redirtied and written back agian through the normal write
> back process.
> 
> Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>

The patch looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
>  fs/ext4/inode.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index fa579e857baf..fc16a89903b9 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -4104,9 +4104,13 @@ static int __ext4_block_zero_page_range(handle_t *handle,
>  	if (ext4_should_journal_data(inode)) {
>  		err = ext4_dirty_journalled_data(handle, bh);
>  	} else {
> -		err = 0;
>  		mark_buffer_dirty(bh);
> -		if (ext4_should_order_data(inode))
> +		/*
> +		 * Only the written block requires ordered data to prevent
> +		 * exposing stale data.
> +		 */
> +		if (!buffer_unwritten(bh) && !buffer_delay(bh) &&
> +		    ext4_should_order_data(inode))
>  			err = ext4_jbd2_inode_add_write(handle, inode, from,
>  					length);
>  	}
> -- 
> 2.52.0
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR