linux-ext4 - mounting with lazytime doesn't work on ext4

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Date:   Mon, 18 Sep 2017 21:26:44 +0200
From:   Markus Trippelsdorf <markus@...ppelsdorf.de>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-ext4@...r.kernel.org
Subject: mounting with lazytime doesn't work on ext4

I switched back to ext4 yesterday, because my btrfs fs got corrupted.
However mounting with lazytime doesn't work, neither specifying it in
/etc/fstab nor a manual remount. It looks like the option is simply
ignored.

Strace shows, e.g.:

 # mount -o lazytime /boot
mount("/dev/sdc2", "/boot", "ext4", MS_LAZYTIME, NULL) = 0 
EXT4-fs (sdc2): mounted filesystem with ordered data mode. Opts: (null) 
/dev/sdc2 on /boot type ext4 (rw,relatime,data=ordered)

 # mount -o remount,lazytime /var
mount("/dev/sdb2", "/var", 0x12c4460, MS_REMOUNT|MS_NOATIME|MS_LAZYTIME, NULL) = 0 
EXT4-fs (sdb2): re-mounted. Opts: (null)
/dev/sdb2 on /var type ext4 (rw,noatime,data=ordered)

When I set "sb->s_flags |= MS_LAZYTIME;" unconditionally in
fs/ext4/super.c:5057 (just deleting the if statement), then lazytime
gets used when I remount.

I'm running the latest git tree (4.14.0-rc1).

-- 
Markus