Re: [PATCH 1/4] xfs: xrep_findroot_block should reject root blocks with siblings

[Brian Foster <bfoster@redhat.com>]

On Tue, Aug 07, 2018 at 08:57:06PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> In xrep_findroot_block, if we find a candidate root block with sibling
> pointers or sibling blocks on the same tree level, we should not return
> that block as a tree root because root blocks cannot have siblings.
> 
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
>  fs/xfs/scrub/repair.c |   17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> 
> diff --git a/fs/xfs/scrub/repair.c b/fs/xfs/scrub/repair.c
> index 85b048b341a0..6c199e2ebb81 100644
> --- a/fs/xfs/scrub/repair.c
> +++ b/fs/xfs/scrub/repair.c
> @@ -727,6 +727,23 @@ xrep_findroot_block(
>  	bp->b_ops->verify_read(bp);
>  	if (bp->b_error)
>  		goto out;
> +
> +	/* Root blocks can't have siblings. */
> +	if (btblock->bb_u.s.bb_leftsib != cpu_to_be32(NULLAGBLOCK) ||
> +	    btblock->bb_u.s.bb_rightsib != cpu_to_be32(NULLAGBLOCK))
> +		goto out;
> +
> +	/*
> +	 * If we find a second block at this level, ignore this level because
> +	 * it can't possibly be a root level.  Maybe we'll find a higher level,
> +	 * or maybe the rmap information is garbage.
> +	 */
> +	if (fab->root != NULLAGBLOCK &&
> +	    fab->height == xfs_btree_get_level(btblock) + 1) {
> +		fab->root = NULLAGBLOCK;
> +		goto out;
> +	}

Ok, but is this enough? Won't resetting fab->root like this mean that
we'd just reassign it to the next block we find at this level? I'm
wondering if we should maintain ->height independently and anticipate
that (height == <valid> && root == NULLAGBLOCK) means we couldn't find a
valid root. That may also allow for more efficient height filtering
during the query.

Brian

> +
>  	fab->root = agbno;
>  	fab->height = xfs_btree_get_level(btblock) + 1;
>  	*found_it = true;
> 
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