linux-ext4 - Re: Lazytime undone by/not working with remount?

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Date:	Mon, 18 May 2015 17:36:52 +0200
From:	Holger Hoffstätte 
	<holger.hoffstaette@...glemail.com>
To:	Eric Sandeen <sandeen@...hat.com>, Karel Zak <kzak@...hat.com>
CC:	linux-ext4@...r.kernel.org, Theodore Ts'o <tytso@....edu>
Subject: Re: Lazytime undone by/not working with remount?


On 05/18/15 17:18, Eric Sandeen wrote:
> On 5/18/15 3:21 AM, Karel Zak wrote:
>> On Sat, May 16, 2015 at 04:26:33PM +0200, Holger Hoffstätte wrote:
>>>> playing with lazytime on 4.0.4-rc1 + yesterday's fencepost patch) I noticed
>>>> something odd. Mounting secondary (non-root) partitions with lazytime works
>>>> fine, but / does not seem to retain the value from fstab - apparently because
>>>> it is remounted rw during boot, and lazytime gets swallowed/undone.
>>>>
>>>> Same effect when trying to remount manually with lazytime:
>>>>
>>>> tux>findmnt /
>>>> TARGET SOURCE    FSTYPE OPTIONS
>>>> /      /dev/sda1 ext4   rw,noatime
>>>>
>>>> tux>mount -o lazytime,remount / 
>>>>
>>>> tux>dmesg 
>>>> [ 5208.482505] EXT4-fs (sda1): re-mounted. Opts: (null)
>>>>
>>>> tux>findmnt /                  
>>>> TARGET SOURCE    FSTYPE OPTIONS
>>>> /      /dev/sda1 ext4   rw,noatime
>>>>
>>>> tux>mount --version
>>>> mount from util-linux 2.26.2 (libmount 2.26.0: assert, debug)
> 
> And what does /proc/mounts say?  That'll tell you what is actually set
> on the superblock.  Works here, on 4.1.0-rc2:
> 
> # mount /dev/sdb1 /mnt/test
> # mount -o remount,lazytime /mnt/test
> # grep sdb1 /proc/mounts 
> /dev/sdb1 /mnt/test ext4 rw,lazytime,seclabel,relatime,data=ordered 0 0
> 
> # dmesg | tail -n 2
> [516203.450943] EXT4-fs (sdb1): mounted filesystem with ordered data mode. Opts: (null)
> [516211.222020] EXT4-fs (sdb1): re-mounted. Opts: lazytime

When I used util-linux 2.26.2 /proc/mounts never contained lazytime for the root device (sda1), despite the fact that it and other partitions explicitly had lazytime in fstab. Secondary drives & partitions *did* get the value right from the start, i.e. anything that didn't go through a ro->remount transition.

It all works reliably with 2.25.x since - as Karel mentioned - the bug seems with ext4's remount logic in combination with readonly (as was the case with the root partition) and mount now actually sending the MS_LAZYTIME flag, instead of relying on ext4's builtin extra handling.

-h

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