xfs: fix off-by-one error when the last rt extent is in use

The fsmap implementation for realtime devices uses the gap between
info->next_daddr and a free rtextent reported by xfs_rtalloc_query_range
to feed userspace fsmap records with an "unknown" owner.  We use this
trick to report to userspace when the last rtextent in the filesystem is
in use by synthesizing a null rmap record starting at the next block
after the query range.

Unfortunately, there's a minor accounting bug in the way that we
construct the null rmap record.  Originally, ahigh.ar_startext contains
the last rtextent for which the user wants records.  It's entirely
possible that number is beyond the end of the rt volume, so the location
synthesized rmap record /must/ be constrained to the minimum of the high
key and the number of extents in the rt volume.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>

fs/xfs/xfs_fsmap.c

@@ -523,27 +523,37 @@ xfs_getfsmap_rtdev_rtbitmap_query(
 {
 	struct xfs_rtalloc_rec		alow = { 0 };
 	struct xfs_rtalloc_rec		ahigh = { 0 };
+	struct xfs_mount		*mp = tp->t_mountp;
 	int				error;
 
-	xfs_ilock(tp->t_mountp->m_rbmip, XFS_ILOCK_SHARED);
+	xfs_ilock(mp->m_rbmip, XFS_ILOCK_SHARED);
 
+	/*
+	 * Set up query parameters to return free rtextents covering the range
+	 * we want.
+	 */
 	alow.ar_startext = info->low.rm_startblock;
 	ahigh.ar_startext = info->high.rm_startblock;
-	do_div(alow.ar_startext, tp->t_mountp->m_sb.sb_rextsize);
-	if (do_div(ahigh.ar_startext, tp->t_mountp->m_sb.sb_rextsize))
+	do_div(alow.ar_startext, mp->m_sb.sb_rextsize);
+	if (do_div(ahigh.ar_startext, mp->m_sb.sb_rextsize))
 		ahigh.ar_startext++;
 	error = xfs_rtalloc_query_range(tp, &alow, &ahigh,
 			xfs_getfsmap_rtdev_rtbitmap_helper, info);
 	if (error)
 		goto err;
 
-	/* Report any gaps at the end of the rtbitmap */
+	/*
+	 * Report any gaps at the end of the rtbitmap by simulating a null
+	 * rmap starting at the block after the end of the query range.
+	 */
 	info->last = true;
+	ahigh.ar_startext = min(mp->m_sb.sb_rextents, ahigh.ar_startext);
+
 	error = xfs_getfsmap_rtdev_rtbitmap_helper(tp, &ahigh, info);
 	if (error)
 		goto err;
 err:
-	xfs_iunlock(tp->t_mountp->m_rbmip, XFS_ILOCK_SHARED);
+	xfs_iunlock(mp->m_rbmip, XFS_ILOCK_SHARED);
 	return error;
 }