Re: mounting with lazytime doesn't work on ext4

[Markus Trippelsdorf <markus@trippelsdorf.de>]

On 2017.09.19 at 13:00 +0200, Lukas Czerner wrote:
> On Mon, Sep 18, 2017 at 09:26:44PM +0200, Markus Trippelsdorf wrote:
> > 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).
> 
> Hi,
> 
> I can't really see the problem on my system (4.14.0-rc1)
> 
> # mount -o lazytime /dev/vdb /mnt/test
> # mount | grep vdb
> /dev/vdb on /mnt/test type ext4 (rw,relatime,lazytime,seclabel,data=ordered)
> 
> Log says:
> EXT4-fs (vdb): mounted filesystem with ordered data mode. Opts: lazytime
> 
> or even via remount
> 
> # mount /dev/vdb /mnt/test
> # mount | grep vdb
> /dev/vdb on /mnt/test type ext4 (rw,relatime,seclabel,data=ordered)
> # mount -o remount,lazytime /mnt/test
> # mount | grep vdb
> /dev/vdb on /mnt/test type ext4 (rw,relatime,lazytime,seclabel,data=ordered)
> 
> Log says:
> EXT4-fs (vdb): re-mounted. Opts: data=ordered,lazytime
> 
> This is probably because I have older version of util-linux that did
> now knew about MS_LAZYTIME so it passed "lazytime" option down to the
> filesystem. So we're definitelly missing something in kernel for this to
> work properly.
> 
> Indeed with newer version of util-linux, specifically the one containing
> commit v2.26-169-g8c7f073 I can reproduce.

Yes. It took me some time to figure out that this is a very recent Linux
regression (since commit e462ec50cb5). I've have send a simple patch to
fix the issue.

-- 
Markus