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

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Date:   Tue, 19 Jul 2022 09:51:33 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     tytso@....edu, adilger.kernel@...ger.ca
Cc:     linux-ext4@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        lczerner@...hat.com, Benjamin Coddington <bcodding@...hat.com>
Subject: should we make "-o iversion" the default on ext4 ?

Back in 2018, I did a patchset [1] to rework the inode->i_version
counter handling to be much less expensive, particularly when no-one is
querying for it.

Testing at the time showed that the cost of enabling i_version on ext4
was close to 0 when nothing is querying it, but I stopped short of
trying to make it the default at the time (mostly out of an abundance of
caution). Since then, we still see a steady stream of cache-coherency
problems with NFSv4 on ext4 when this option is disabled (e.g. [2]).

Is it time to go ahead and make this option the default on ext4? I don't
see a real downside to doing so, though I'm unclear on how we should
approach this. Currently the option is twiddled using MS_I_VERSION flag,
and it's unclear to me how we can reverse the sense of such a flag.

Thoughts?

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a4b7fd7d34de5765dece2dd08060d2e1f7be3b39
[2]: https://bugzilla.redhat.com/show_bug.cgi?id=2107587

-- 
Jeff Layton <jlayton@...nel.org>