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

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Message-ID: <20170919110021.3yn6n4ot3whh6myi@rh_laptop>
Date:   Tue, 19 Sep 2017 13:00:21 +0200
From:   Lukas Czerner <lczerner@...hat.com>
To:     Markus Trippelsdorf <markus@...ppelsdorf.de>
Cc:     Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-ext4@...r.kernel.org
Subject: Re: mounting with lazytime doesn't work on ext4

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.

Thanks!
-Lukas

> 
> -- 
> Markus