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

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Date:   Wed, 17 Jun 2020 23:33:33 -0400
From:   Masayoshi Mizuma <msys.mizuma@...il.com>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     "J. Bruce Fields" <bfields@...ldses.org>,
        Eric Sandeen <sandeen@...deen.net>,
        Christoph Hellwig <hch@...radead.org>,
        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 06:44:29PM -0700, Darrick J. Wong wrote:
> On Wed, Jun 17, 2020 at 09:30:26PM -0400, Masayoshi Mizuma wrote:
> > On Wed, Jun 17, 2020 at 02:45:07PM -0400, J. Bruce Fields wrote:
> > > On Wed, Jun 17, 2020 at 01:28:11PM -0500, Eric Sandeen wrote:
> > > > but mount(8) has already exposed this interface:
> > > > 
> > > >        iversion
> > > >               Every time the inode is modified, the i_version field will be incremented.
> > > > 
> > > >        noiversion
> > > >               Do not increment the i_version inode field.
> > > > 
> > > > so now what?
> > > 
> > > It's not like anyone's actually depending on i_version *not* being
> > > incremented.  (Can you even observe it from userspace other than over
> > > NFS?)
> > > 
> > > So, just silently turn on the "iversion" behavior and ignore noiversion,
> > > and I doubt you're going to break any real application.
> > 
> > I suppose it's probably good to remain the options for user compatibility,
> > however, it seems that iversion and noiversiont are useful for
> > only ext4.
> > How about moving iversion and noiversion description on mount(8)
> > to ext4 specific option?
> > 
> > And fixing the remount issue for XFS (maybe btrfs has the same
> > issue as well)?
> > For XFS like as:
> > 
> > diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> > index 379cbff438bc..2ddd634cfb0b 100644
> > --- a/fs/xfs/xfs_super.c
> > +++ b/fs/xfs/xfs_super.c
> > @@ -1748,6 +1748,9 @@ xfs_fc_reconfigure(
> >                         return error;
> >         }
> > 
> > +       if (XFS_SB_VERSION_NUM(&mp->m_sb) == XFS_SB_VERSION_5)
> > +               mp->m_super->s_flags |= SB_I_VERSION;
> > +
> 
> I wonder, does this have to be done at the top of this function because
> the vfs already removed S_I_VERSION from s_flags?

Ah, I found the above code doesn't work...
sb->s_flags will be updated after reconfigure():

int reconfigure_super(struct fs_context *fc)
{
...
        if (fc->ops->reconfigure) {
                retval = fc->ops->reconfigure(fc);
                if (retval) {
                        if (!force)
                                goto cancel_readonly;
                        /* If forced remount, go ahead despite any errors */
                        WARN(1, "forced remount of a %s fs returned %i\n",
                             sb->s_type->name, retval);
                }
        }

        WRITE_ONCE(sb->s_flags, ((sb->s_flags & ~fc->sb_flags_mask) |
                                 (fc->sb_flags & fc->sb_flags_mask)));

Here, fc->sb_flags_mask should be MS_RMT_MASK, so SB_I_VERSION will be
dropped except iversion mount opstion (MS_I_VERSION) is set.

- Masa