When an exceptional event triggers xfs_force_shutdown() tear down dax
mappings.  Quoting Dave,

    "The simple fact is that a /filesystem/ shutdown needs to do DAX
    mapping invalidation regardless of whether the block device has
    been unplugged or not. This is not a case of "this only happens
    when we unplug the device", this is a user data protection
    mechanism that we use to prevent corruption propagation once it
    has been detected. A device unplug is just one type of
    "corruption" that can occur."

Signed-off-by: Dan Williams <redacted>
---
 fs/xfs/xfs_fsops.c |    9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/xfs/xfs_fsops.c b/fs/xfs/xfs_fsops.c
index ee3aaa0a5317..0c6a52809dcc 100644
--- a/fs/xfs/xfs_fsops.c
+++ b/fs/xfs/xfs_fsops.c

@@ -828,6 +828,15 @@ xfs_do_force_shutdown(
 	if (xfs_log_force_umount(mp, logerror))
 		return;
 
+	/*
+	 * If DAX is in use, we have to unmap all direct access virtual
+	 * mappings to ensure nothing more gets written directly from
+	 * userspace. This will force them to refault and that will
+	 * result in them detecting the shutdown condition and hence
+	 * will fail appropriately.
+	 */
+	unmap_dax_inodes(mp->m_super);
+
 	if (flags & SHUTDOWN_CORRUPT_INCORE) {
 		xfs_alert_tag(mp, XFS_PTAG_SHUTDOWN_CORRUPT,
     "Corruption of in-memory data detected.  Shutting down filesystem");