Re: [PATCH V2 12/23] xfs_db: Add support to read from external log device

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Jun 06, 2023 at 02:57:55PM +0530, Chandan Babu R wrote:
> This commit introduces a new function set_log_cur() allowing xfs_db to read
> from an external log device. This is required by a future commit which will
> add the ability to dump metadata from external log devices.
> 
> Signed-off-by: Chandan Babu R <chandan.babu@oracle.com>
> ---
>  db/io.c | 57 ++++++++++++++++++++++++++++++++++++++++++++-------------
>  1 file changed, 44 insertions(+), 13 deletions(-)
> 
> diff --git a/db/io.c b/db/io.c
> index 3d257236..a6feaba3 100644
> --- a/db/io.c
> +++ b/db/io.c
> @@ -508,18 +508,19 @@ write_cur(void)
>  
>  }
>  
> -void
> -set_cur(
> -	const typ_t	*type,
> -	xfs_daddr_t	blknum,
> -	int		len,
> -	int		ring_flag,
> -	bbmap_t		*bbmap)
> +static void
> +__set_cur(
> +	struct xfs_buftarg	*btargp,
> +	const typ_t		*type,
> +	xfs_daddr_t		 blknum,
> +	int			 len,
> +	int			 ring_flag,
> +	bbmap_t			*bbmap)
>  {
> -	struct xfs_buf	*bp;
> -	xfs_ino_t	dirino;
> -	xfs_ino_t	ino;
> -	uint16_t	mode;
> +	struct xfs_buf		*bp;
> +	xfs_ino_t		dirino;
> +	xfs_ino_t		ino;
> +	uint16_t		mode;
>  	const struct xfs_buf_ops *ops = type ? type->bops : NULL;
>  	int		error;
>  
> @@ -548,11 +549,11 @@ set_cur(
>  		if (!iocur_top->bbmap)
>  			return;
>  		memcpy(iocur_top->bbmap, bbmap, sizeof(struct bbmap));
> -		error = -libxfs_buf_read_map(mp->m_ddev_targp, bbmap->b,
> +		error = -libxfs_buf_read_map(btargp, bbmap->b,
>  				bbmap->nmaps, LIBXFS_READBUF_SALVAGE, &bp,
>  				ops);
>  	} else {
> -		error = -libxfs_buf_read(mp->m_ddev_targp, blknum, len,
> +		error = -libxfs_buf_read(btargp, blknum, len,
>  				LIBXFS_READBUF_SALVAGE, &bp, ops);
>  		iocur_top->bbmap = NULL;
>  	}
> @@ -589,6 +590,36 @@ set_cur(
>  		ring_add();
>  }
>  
> +void
> +set_cur(
> +	const typ_t	*type,
> +	xfs_daddr_t	blknum,
> +	int		len,
> +	int		ring_flag,
> +	bbmap_t		*bbmap)
> +{
> +	__set_cur(mp->m_ddev_targp, type, blknum, len, ring_flag, bbmap);
> +}
> +
> +void
> +set_log_cur(
> +	const typ_t	*type,
> +	xfs_daddr_t	blknum,
> +	int		len,
> +	int		ring_flag,
> +	bbmap_t		*bbmap)
> +{
> +	struct xfs_buftarg	*btargp = mp->m_ddev_targp;
> +
> +	if (mp->m_logdev_targp->bt_bdev != mp->m_ddev_targp->bt_bdev) {
> +		ASSERT(mp->m_sb.sb_logstart == 0);
> +		btargp = mp->m_logdev_targp;
> +	}

I think this should print an error message if there isn't an external
log device and then leave iocur_top unchanged:

	if (mp->m_logdev_targp->bt_bdev == mp->m_ddev_targp->bt_bdev) {
		fprintf(stderr, "no external log specified\n");
		exitcode = 1;
		return;
	}

	__set_cur(mp->m_logdev_targp, type, blknum, len, ring_flag, bbmap);

because metadump shouldn't crash if there's an external log device but
sb_logstart is zero.  The superblock fields /could/ be corrupt, or other
weird shenanigans are going on.

(Also it's a layering violation for the io cursors to know anything at
all about the filesystem stored above them.)

--D

> +
> +	__set_cur(btargp, type, blknum, len, ring_flag, bbmap);
> +}
> +
> +
>  void
>  set_iocur_type(
>  	const typ_t	*type)
> -- 
> 2.39.1
>