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

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Date:   Wed, 17 Jun 2020 14:18:16 -0400
From:   "J. Bruce Fields" <bfields@...ldses.org>
To:     Eric Sandeen <sandeen@...deen.net>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        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 12:55:24PM -0500, Eric Sandeen wrote:
> On 6/17/20 12:24 PM, Darrick J. Wong wrote:
> > 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
> 
> unless you're ext4:
> 
> {Opt_i_version, "i_version"},
> 
> ok "iversion" is what mount(8) takes and translates into MS_I_VERSION (thanks Darrick)
> 
> # strace -vv -emount mount -oloop,iversion fsfile mnt
> mount("/dev/loop0", "/tmp/mnt", "xfs", MS_I_VERSION, NULL) = 0
> 
> FWIW, mount actually seems to pass what it finds in /proc/mounts back in on remount for ext4:
> 
> # strace -vv -emount mount -o remount mnt
> mount("/dev/loop0", "/tmp/mnt", 0x55bfcbdca150, MS_REMOUNT|MS_RELATIME, "seclabel,i_version,data=ordered") = 0
> 
> but it still looks unhandled on remount.  Perhaps if /proc/mounts exposed
> "iversion" (not "i_version") then mount -o remount would DTRT.

I'd rather just eliminate the option, to the extent possible.

It was only ever a mount option since it caused a performance regression
in some filesystems, but I *think* that was addressed by Jeff Layton's
work (f02a9ad1f15d "fs: handle inode->i_version more efficiently").

XFS in particular is just using this flag to tell knfsd that it should
use i_version.  I don't think it was really intended for userspace to be
able to turn this off.

--b.