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Description of problem:

[root@el7a ~]# man mount

FILESYSTEM INDEPENDENT MOUNT OPTIONS

       The  following  options  apply to any filesystem that is being mounted (but not every filesystem actually honors them - e.g., the sync option
       today has effect only for ext2, ext3, fat, vfat and ufs):

BUGS

       Some Linux filesystems don't support -o sync and -o dirsync (the ext2, ext3, fat and vfat filesystems do support synchronous  updates  (a  la
       BSD) when mounted with the sync option).

Version-Release number of selected component (if applicable):

[root@el7a ~]# which mount
/usr/bin/mount
[root@el7a ~]# rpm -qf /usr/bin/mount
util-linux-2.23.2-43.el7_4.2.x86_64
[root@el7a ~]# rpm -ql util-linux-2.23.2-43.el7_4.2.x86_64 | grep man8/mount
/usr/share/man/man8/mount.8.gz

Additional info:

Same on RHEL6, Fedora 27

https://access.redhat.com/solutions/322213 says to mount ext4 with sync option for direct I/O

Well, it seems ext4 is missing on some places in the man page.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2018:3178