Re: [PATCH v2 RFC] f2fs: fix to force keeping write barrier for strict fsync mode

[Jaegeuk Kim]

On 06/01, Chao Yu wrote:
> [1] https://www.mail-archive.com/linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx/msg15126.html
> 
> As [1] reported, if lower device doesn't support write barrier, in below
> case:
> 
> - write page #0; persist
> - overwrite page #0
> - fsync
>  - write data page #0 OPU into device's cache
>  - write inode page into device's cache
>  - issue flush

Well, we have preflush for node writes, so I don't think this is the case.

 fio.op_flags |= REQ_PREFLUSH | REQ_FUA;

> 
> If SPO is triggered during flush command, inode page can be persisted
> before data page #0, so that after recovery, inode page can be recovered
> with new physical block address of data page #0, however there may
> contains dummy data in new physical block address.
> 
> Then what user will see is: after overwrite & fsync + SPO, old data in
> file was corrupted, if any user do care about such case, we can suggest
> user to use STRICT fsync mode, in this mode, we will force to trigger
> preflush command to persist data in device cache in prior to node
> writeback, it avoids potential data corruption during fsync().
> 
> Signed-off-by: Chao Yu <yuchao0@xxxxxxxxxx>
> ---
> v2:
> - fix this by adding additional preflush command rather than using
> atomic write flow.
>  fs/f2fs/file.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> index 7d5311d54f63..238ca2a733ac 100644
> --- a/fs/f2fs/file.c
> +++ b/fs/f2fs/file.c
> @@ -301,6 +301,20 @@ static int f2fs_do_sync_file(struct file *file, loff_t start, loff_t end,
>  				f2fs_exist_written_data(sbi, ino, UPDATE_INO))
>  			goto flush_out;
>  		goto out;
> +	} else {
> +		/*
> +		 * for OPU case, during fsync(), node can be persisted before
> +		 * data when lower device doesn't support write barrier, result
> +		 * in data corruption after SPO.
> +		 * So for strict fsync mode, force to trigger preflush to keep
> +		 * data/node write order to avoid potential data corruption.
> +		 */
> +		if (F2FS_OPTION(sbi).fsync_mode == FSYNC_MODE_STRICT &&
> +								!atomic) {
> +			ret = f2fs_issue_flush(sbi, inode->i_ino);
> +			if (ret)
> +				goto out;
> +		}
>  	}
>  go_write:
>  	/*
> -- 
> 2.29.2