On Fri, Aug 13, 2021 at 04:20:34PM +0530, Chandan Babu R wrote:

On 11 Aug 2021 at 17:58, "Darrick J. Wong" [off-list ref] wrote:

quoted

From: Darrick J. Wong <djwong@kernel.org>

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.

When the number of blocks on the realtime device is not an integral multiple
of xfs_sb->sb_rextsize, ahigh.ar_startext can contain a value which is one
more than the highest valid rtextent. Hence, without this patch, the last
record reported to the userpace might contain an invalid upper bound. Assuming
that my understanding is indeed correct,

Correct.  Thanks for reviewing this!

--D

Reviewed-by: Chandan Babu R <redacted>

quoted

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
---
 fs/xfs/xfs_fsmap.c |   22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/fs/xfs/xfs_fsmap.c b/fs/xfs/xfs_fsmap.c
index 7d0b09c1366e..a0e8ab58124b 100644
--- a/fs/xfs/xfs_fsmap.c
+++ b/fs/xfs/xfs_fsmap.c

@@ -523,27 +523,39 @@ 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 extents covering the range we
+	 * want.
+	 */
 	alow.ar_startext = info->low.rm_startblock;
+	do_div(alow.ar_startext, mp->m_sb.sb_rextsize);
+
 	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))
+	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;
 }
 

-- 
chandan