xfs: conditionally allow FS_XFLAG_REALTIME changes if S_DAX is set

If a file has the S_DAX flag (aka fsdax access mode) set, we cannot
allow users to change the realtime flag unless the datadev and rtdev
both support fsdax access modes.  Even if there are no extents allocated
to the file, the setattr thread could be racing with another thread
that has already started down the write code paths.

Fixes: ba23cba9 ("fs: allow per-device dax status checking for filesystems")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>

fs/xfs/xfs_ioctl.c

@@ -483,6 +483,17 @@ xfs_ioctl_setattr_xflags(
 		/* Can't change realtime flag if any extents are allocated. */
 		if (ip->i_df.if_nextents || ip->i_delayed_blks)
 			return -EINVAL;
+
+		/*
+		 * If S_DAX is enabled on this file, we can only switch the
+		 * device if both support fsdax.  We can't update S_DAX because
+		 * there might be other threads walking down the access paths.
+		 */
+		if (IS_DAX(VFS_I(ip)) &&
+		    (mp->m_ddev_targp->bt_daxdev == NULL ||
+		     (mp->m_rtdev_targp &&
+		      mp->m_rtdev_targp->bt_daxdev == NULL)))
+			return -EINVAL;
 	}
 
 	if (rtflag) {