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

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Date:   Thu, 18 Jun 2020 22:20:05 -0400
From:   "J. Bruce Fields" <bfields@...ldses.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Masayoshi Mizuma <msys.mizuma@...il.com>,
        Eric Sandeen <sandeen@...deen.net>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        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>, jlayton@...hat.com
Subject: Re: [PATCH] fs: i_version mntopt gets visible through /proc/mounts

On Fri, Jun 19, 2020 at 08:39:48AM +1000, Dave Chinner wrote:
> On Wed, Jun 17, 2020 at 11:45:35PM -0400, Masayoshi Mizuma wrote:
> > Thank you for pointed it out.
> > How about following change? I believe it works both xfs and btrfs...
> > 
> > diff --git a/fs/super.c b/fs/super.c
> > index b0a511bef4a0..42fc6334d384 100644
> > --- a/fs/super.c
> > +++ b/fs/super.c
> > @@ -973,6 +973,9 @@ int reconfigure_super(struct fs_context *fc)
> >                 }
> >         }
> > 
> > +       if (sb->s_flags & SB_I_VERSION)
> > +               fc->sb_flags |= MS_I_VERSION;
> > +
> >         WRITE_ONCE(sb->s_flags, ((sb->s_flags & ~fc->sb_flags_mask) |
> >                                  (fc->sb_flags & fc->sb_flags_mask)));
> >         /* Needs to be ordered wrt mnt_is_readonly() */
> 
> This will prevent SB_I_VERSION from being turned off at all. That
> will break existing filesystems that allow SB_I_VERSION to be turned
> off on remount, such as ext4.
> 
> The manipulations here need to be in the filesystem specific code;
> we screwed this one up so badly there is no "one size fits all"
> behaviour that we can implement in the generic code...

My memory was that after Jeff Layton's i_version patches, there wasn't
really a significant performance hit any more, so the ability to turn it
off is no longer useful.

But looking back through Jeff's postings, I don't see him claiming that;
e.g. in:

	https://lore.kernel.org/lkml/20171222120556.7435-1-jlayton@kernel.org/
	https://lore.kernel.org/linux-nfs/20180109141059.25929-1-jlayton@kernel.org/
	https://lore.kernel.org/linux-nfs/1517228795.5965.24.camel@redhat.com/

he reports comparing old iversion behavior to new iversion behavior, but
not new iversion behavior to new noiversion behavior.

--b.