linux-ext4 - Re: should we make "-o iversion" the default on ext4 ?

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Date:   Wed, 20 Jul 2022 12:29:12 -0400
From:   "Benjamin Coddington" <bcodding@...hat.com>
To:     "Jeff Layton" <jlayton@...nel.org>
Cc:     "Lukas Czerner" <lczerner@...hat.com>, tytso@....edu,
        adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: should we make "-o iversion" the default on ext4 ?

On 20 Jul 2022, at 12:15, Jeff Layton wrote:

> On Wed, 2022-07-20 at 11:56 -0400, Benjamin Coddington wrote:
>> On 20 Jul 2022, at 10:38, Jeff Layton wrote:
>>> On Wed, 2022-07-20 at 16:15 +0200, Lukas Czerner wrote:
>>>>
>>>> Is there a different way I am not seeing?
>>>>
>>>
>>> Right, implementing this is the difficult bit actually since this uses a
>>> MS_* flag. If we do make this the default, we'd definitely want to
>>> continue allowing "-o noiversion" to disable it.
>>>
>>> Could we just reverse the default in libmount? It might cause this to
>>> suddenly be enabled in some deployments, but in most cases, people
>>> wouldn't even notice and they could still specify -o noiversion to turn
>>> it off.
>>>
>>> Another idea would be to introduce new mount options for this, but
>>> that's kind of nasty from a UI standpoint.
>>
>> Is it safe to set SB_I_VERSION at export time?  If so, export_operations
>> could grow an ->enable_iversion().
>>
>
> That sounds like it might be problematic.
>
> Consider the case where a NFSv4 client has cached file data and the
> change attribute for the file. Server then reboots, but before the
> export happens a local user makes a change to the file and it doesn't
> update the i_version.

Nfsd currently uses both ctime and i_version if its available, I'd expect
that eliminates this case.