xfs: Show realtime device stats on statfs calls if realtime flags set

- Reports realtime device free blocks in statfs calls if (realtime)
  inheritance bit is set on the inode of directory, or realtime flag
  in the case of files.  This is a bit more intuitive, especially for
  use-cases which are using a much larger device for the realtime device.
- Add XFS_IS_REALTIME_MOUNT option to gate based on the existence of a
  realtime device on the mount, similar to the XFS_IS_REALTIME_INODE
  option.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Richard Wareing <rwareing@fb.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/xfs_linux.h

@@ -285,8 +285,10 @@ static inline uint64_t howmany_64(uint64_t x, uint32_t y)
 #define XFS_IS_REALTIME_INODE(ip)			\
 	(((ip)->i_d.di_flags & XFS_DIFLAG_REALTIME) &&	\
 	 (ip)->i_mount->m_rtdev_targp)
+#define XFS_IS_REALTIME_MOUNT(mp) ((mp)->m_rtdev_targp ? 1 : 0)
 #else
 #define XFS_IS_REALTIME_INODE(ip) (0)
+#define XFS_IS_REALTIME_MOUNT(mp) (0)
 #endif
 
 #endif /* __XFS_LINUX__ */

fs/xfs/xfs_super.c

@@ -1153,6 +1153,14 @@ xfs_fs_statfs(
 	    ((mp->m_qflags & (XFS_PQUOTA_ACCT|XFS_PQUOTA_ENFD))) ==
 			      (XFS_PQUOTA_ACCT|XFS_PQUOTA_ENFD))
 		xfs_qm_statvfs(ip, statp);
+
+	if (XFS_IS_REALTIME_MOUNT(mp) &&
+	    (ip->i_d.di_flags & (XFS_DIFLAG_RTINHERIT | XFS_DIFLAG_REALTIME))) {
+		statp->f_blocks = sbp->sb_rblocks;
+		statp->f_bavail = statp->f_bfree =
+			sbp->sb_frextents * sbp->sb_rextsize;
+	}
+
 	return 0;
 }