Re: [PATCH 21/63] xfs: map an inode's offset to an exact physical block

+ * For a remap operation, just "allocate" an extent at the address that the
+ * caller passed in, and ensure that the AGFL is the right size.  The caller
+ * will then map the "allocated" extent into the file somewhere.
+ */
+STATIC int
+xfs_bmap_remap_alloc(
+	struct xfs_bmalloca	*ap)
+{
+	struct xfs_trans	*tp = ap->tp;
+	struct xfs_mount	*mp = tp->t_mountp;
+	xfs_agblock_t		bno;
+	struct xfs_alloc_arg	args;
+	int			error;
+
+	/*
+	 * validate that the block number is legal - the enables us to detect
+	 * and handle a silent filesystem corruption rather than crashing.
+	 */
+	memset(&args, 0, sizeof(struct xfs_alloc_arg));
+	args.tp = ap->tp;
+	args.mp = ap->tp->t_mountp;
+	bno = *ap->firstblock;
+	args.agno = XFS_FSB_TO_AGNO(mp, bno);
+	ASSERT(args.agno < mp->m_sb.sb_agcount);
+	args.agbno = XFS_FSB_TO_AGBNO(mp, bno);
+	ASSERT(args.agbno < mp->m_sb.sb_agblocks);

Shouldn't this return -EFSCORRUPED instead?  Otherwise the comment
above isn't really true.

Otherwise this looks fine to me:

Reviewed-by: Christoph Hellwig <hch@lst.de>