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

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Date:   Wed, 17 Jun 2020 10:24:56 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Eric Sandeen <sandeen@...deen.net>
Cc:     "J. Bruce Fields" <bfields@...ldses.org>,
        Christoph Hellwig <hch@...radead.org>,
        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,
        linux-xfs <linux-xfs@...r.kernel.org>
Subject: Re: [PATCH] fs: i_version mntopt gets visible through /proc/mounts

On Wed, Jun 17, 2020 at 12:14:28PM -0500, Eric Sandeen wrote:
> 
> 
> On 6/17/20 10:58 AM, J. Bruce Fields wrote:
> > 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.
> 
> Not on XFS AFAICT:
> 
> [600280.685810] xfs: Unknown parameter 'i_version'

Yeah, because the mount option is 'iversion', not 'i_version'.  Even if
you were going to expose the flag state in /proc/mounts, the text string
should match the mount option.

> so we can't be exporting "mount options" for xfs that aren't actually
> going to be accepted by the filesystem.
> 
> > 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?
> 
> <cc xfs list>
> 
> XFS doesn't manipulate that flag on remount.  We just turn it on by default
> for modern filesystem formats:
> 
>         /* version 5 superblocks support inode version counters. */
>         if (XFS_SB_VERSION_NUM(&mp->m_sb) == XFS_SB_VERSION_5)
>                 sb->s_flags |= SB_I_VERSION;
> 
> Also, this behavior doesn't seem unique to xfs:
> 
> # mount -o loop,i_version fsfile test_iversion
> # grep test_iversion /proc/mounts
> /dev/loop4 /tmp/test_iversion ext4 rw,seclabel,relatime,i_version 0 0
> # mount -o remount test_iversion
> # grep test_iversion /proc/mounts
> /dev/loop4 /tmp/test_iversion ext4 rw,seclabel,relatime 0 0
> # uname -a
> Linux <hostname> 5.7.0-rc4+ #7 SMP Wed Jun 10 14:01:34 EDT 2020 x86_64 x86_64 x86_64 GNU/Linux

Probably because do_mount clears it and I guess xfs don't re-enable
it in any of their remount functions...

--D

> -Eric
> 
> > 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.
> >