linux-ext4 - Re: [PATCH] fs: i_version mntopt gets visible through /proc/mounts

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Date:   Wed, 17 Jun 2020 11:58:36 -0400
From:   bfields@...ldses.org (J. Bruce Fields)
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Masayoshi Mizuma <msys.mizuma@...il.com>,
        Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Masayoshi Mizuma <m.mizuma@...fujitsu.com>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs: i_version mntopt gets visible through /proc/mounts

On Wed, Jun 17, 2020 at 01:03:14AM -0700, Christoph Hellwig wrote:
> On Tue, Jun 16, 2020 at 04:21:23PM -0400, Masayoshi Mizuma wrote:
> > From: Masayoshi Mizuma <m.mizuma@...fujitsu.com>
> > 
> > /proc/mounts doesn't show 'i_version' even if iversion
> > mount option is set to XFS.
> > 
> > iversion mount option is a VFS option, not ext4 specific option.
> > Move the handler to show_sb_opts() so that /proc/mounts can show
> > 'i_version' on not only ext4 but also the other filesystem.
> 
> SB_I_VERSION is a kernel internal flag.  XFS doesn't have an i_version
> mount option.

It probably *should* be a kernel internal flag, but it seems to work as
a mount option too.

By coincidence I've just been looking at a bug report showing that
i_version support is getting accidentally turned off on XFS whenever
userspace does a read-write remount.

Is there someplace in the xfs mount code where it should be throwing out
SB_I_VERSION?

Ideally there'd be entirely different fields for mount options and
internal feature flags.  But I don't know, maybe SB_I_VERSION is the
only flag we have like this.

--b.