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

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Date:   Wed, 17 Jun 2020 12:14:28 -0500
From:   Eric Sandeen <sandeen@...deen.net>
To:     "J. Bruce Fields" <bfields@...ldses.org>,
        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,
        linux-xfs <linux-xfs@...r.kernel.org>
Subject: Re: [PATCH] fs: i_version mntopt gets visible through /proc/mounts



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'

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

-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.
>