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

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Date:   Fri, 19 Jun 2020 08:04:53 -0400
From:   Jeff Layton <jlayton@...hat.com>
To:     Dave Chinner <david@...morbit.com>,
        "J. Bruce Fields" <bfields@...ldses.org>
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>
Subject: Re: [PATCH] fs: i_version mntopt gets visible through /proc/mounts

On Fri, 2020-06-19 at 12:44 +1000, Dave Chinner wrote:
> On Thu, Jun 18, 2020 at 10:20:05PM -0400, J. Bruce Fields wrote:
> > 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.
> 
> Yes, I completely agree with you here. However, with some
> filesystems allowing it to be turned off, we can't just wave our
> hands and force enable the option. Those filesystems - if the
> maintainers chose to always enable iversion - will have to go
> through a mount option deprecation period before permanently
> enabling it.
>
> > 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.
> 
> Yeah, it's had to compare noiversion behaviour on filesystems where
> it was understood that it couldn't actually be turned off. And,
> realistically, the comaprison to noiversion wasn't really relevant
> to the problem Jeff's patchset was addressing...
> 

I actually did do some comparison with that patchset vs. noiversion
mounted ext4, and found that there was a small performance delta. It
wasn't much but it was measurable enough that I didn't want to propose
removing the option from ext4 altogether at the time. It may be worth it
to do that now though.
-- 
Jeff Layton <jlayton@...hat.com>