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Subject: Re: [RFC PATCH] ext4: add unmount filesystem message
To:     Theodore Ts'o <tytso@mit.edu>,
        Gabriel Krisman Bertazi <krisman@collabora.com>
CC:     <linux-ext4@vger.kernel.org>, <adilger.kernel@dilger.ca>,
        <jack@suse.cz>, <yukuai3@huawei.com>, <yebin10@huawei.com>,
        <liuzhiqiang26@huawei.com>, <liangyun2@huawei.com>
References: <20220412145320.2669897-1-yi.zhang@huawei.com>
 <87pmlmcmu6.fsf@collabora.com> <YlYo/FqujCnUHH6X@mit.edu>
From:   Zhang Yi <yi.zhang@huawei.com>
Message-ID: <fe9fcfcd-7c6c-19eb-525c-f8a79804481c@huawei.com>
Date:   Wed, 13 Apr 2022 10:23:31 +0800
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On 2022/4/13 9:35, Theodore Ts'o wrote:
> On Tue, Apr 12, 2022 at 12:01:37PM -0400, Gabriel Krisman Bertazi wrote:
>> Zhang Yi <yi.zhang@huawei.com> writes:
>>
>>> Now that we have kernel message at mount time, system administrator
> 
> "Now that we have...." is a bit misleading, since (at least to an
> English speaker) that this is something that was recently added, and
> that's not the case.
> 
>>> could acquire the mount time, device and options easily. But we don't
>>> have corresponding unmounting message at umount time, so we cannot know
>>> if someone umount a filesystem easily. Some of the modern filesystems
>>> (e.g. xfs) have the umounting kernel message, so add one for ext4
>>> filesystem for convenience.
>>>
>>>  EXT4-fs (sdb): mounted filesystem with ordered data mode. Quota mode: none.
>>>  EXT4-fs (sdb): unmounting filesystem.
>>
>> I don't think sysadmins should be relying on the kernel log for this,
>> since the information can easily be overwritten by new messages there.
>> Is there a reason why you can't just monitor /proc/self/mountinfo?
> 
> You're right that it can be dangerous for sysadmins to be relying on
> the kernel log for mount and umount notifications --- but it depends
> on what they think it means, and the potential pitfalls are there for
> both the mount and unmount messages.  The problem of course, is that
> bind mounts, and mount name spaces, so if the question is whether a
> file system is available at a particular mount point, then using the
> kernel log is definitely not going to be reliable.
> 
> But if the goal is to determine whether a particular device is safe to
> run fsck or otherwise access directly, or for the purposes of
> debugging the kernel and looking at the logs to understand when the
> device is being accessed by the kernel and when the file system is
> done with the device, I can see how it might be useful.
> 

Yes, I understand that the kernel log is not reliable, and
/proc/self/mountinfo neither. Our goal is simple, As Ted said, just add a
method to help sysadmins to know whether a particular ext4 device is really
doing unmount procedure, it could be helpful for us to debug kernel and
locate kernel bug.

Thanks,
Yi.