Re: [PATCH 6/8] xfs: scrub big block inode btrees correctly

[Brian Foster <bfoster@redhat.com>]

On Mon, Dec 31, 2018 at 06:09:10PM -0800, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> Teach scrub how to handle the case that there are one or more inobt
> records covering a given inode cluster.  This fixes the operation on big
> block filesystems (e.g. 64k blocks, 512 byte inodes).
> 
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
>  fs/xfs/scrub/ialloc.c |    8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> 
> diff --git a/fs/xfs/scrub/ialloc.c b/fs/xfs/scrub/ialloc.c
> index 2f6c2d7fa3fd..1be6a5ebd61e 100644
> --- a/fs/xfs/scrub/ialloc.c
> +++ b/fs/xfs/scrub/ialloc.c
> @@ -162,6 +162,7 @@ xchk_iallocbt_check_cluster_ifree(
>  	xfs_ino_t			fsino;
>  	xfs_agino_t			agino;
>  	unsigned int			offset;
> +	unsigned int			cluster_buf_base;
>  	bool				irec_free;
>  	bool				ino_inuse;
>  	bool				freemask_ok;
> @@ -174,11 +175,14 @@ xchk_iallocbt_check_cluster_ifree(
>  	 * Given an inobt record, an offset of a cluster within the record,
>  	 * and an offset of an inode within a cluster, compute which fs inode
>  	 * we're talking about and the offset of the inode record within the
> -	 * inode buffer.
> +	 * inode buffer, being careful about inobt records that don't align
> +	 * with the start of the inode buffer when block sizes are large.
>  	 */
>  	agino = irec->ir_startino + cluster_base + cluster_index;
>  	fsino = XFS_AGINO_TO_INO(mp, bs->cur->bc_private.a.agno, agino);
> -	offset = cluster_index * mp->m_sb.sb_inodesize;
> +	cluster_buf_base = XFS_INO_TO_OFFSET(mp, irec->ir_startino +
> +					     cluster_base);
> +	offset = (cluster_buf_base + cluster_index) * mp->m_sb.sb_inodesize;

So cluster_buf_base should always be 0 unless the record itself is not
cluster aligned (i.e., the second record in a large FSB), right? If so,
cluster_base should also be 0 in any case where cluster_buf_base != 0.
I'm wondering if that can be made a little more explicit, or perhaps
self-documented with an assert.

Thinking more about it, it's kind of confusing to me either way because
cluster_index isn't really a cluster index in this case, rather it's
more of a record index because it doesn't account for the fact that the
record is a subset of the cluster. Hmmm... could we simplify this by
setting imap.im_boffset appropriately in the caller such that dip always
points to either the first inode in the buffer or first inode in the
record, then just pass in dip directly and let the caller increment it
in the associated loop? Maybe that's something for another patch..

Brian

>  	if (offset >= BBTOB(cluster_bp->b_length)) {
>  		xchk_btree_set_corrupt(bs->sc, bs->cur, 0);
>  		goto out;
>