linux-kernel - Re: Inverted mount options completely broken (iversion,relatime)

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Message-ID: <1f56432b-a245-a010-51fd-814a9cf4e2b1@redhat.com>
Date:   Wed, 29 Jul 2020 11:41:45 -0700
From:   Eric Sandeen <sandeen@...hat.com>
To:     Josef Bacik <josef@...icpanda.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@....de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        David Sterba <dsterba@...e.com>
Subject: Re: Inverted mount options completely broken (iversion,relatime)

On 7/29/20 11:32 AM, Josef Bacik wrote:
> Hello,
> 
> Eric reported a problem to me where we were clearing SB_I_VERSION on remount of a btrfs file system.  After digging through I discovered it's because we expect the proper flags that we want to be passed in via the mount() syscall, and because we didn't have "iversion" in our show_options entry the mount binary (form util-linux) wasn't setting MS_I_VERSION for the remount, and thus the VFS was clearing SB_I_VERSION from our s_flags.
> 
> No big deal, I'll fix show_mount.  Except Eric then noticed that mount -o noiversion didn't do anything, we still get iversion set.  That's because btrfs just defaults to having SB_I_VERSION set.  Furthermore -o noiversion doesn't get sent into mount, it's handled by the mount binary itself, and it does this by not having MS_I_VERSION set in the mount flags.

This was beaten^Wdiscussed to death in an earlier thread,
[PATCH] fs: i_version mntopt gets visible through /proc/mounts

https://lore.kernel.org/linux-fsdevel/20200616202123.12656-1-msys.mizuma@gmail.com/

tl;dr: hch doesn't think [no]iversion should be exposed as an option /at all/
so exposing it in /proc/mounts in show_mnt_opts for mount(8)'s benefit was
nacked.

> This happens as well for -o relatime, it's the default and so if you do mount -o norelatime it won't do anything, you still get relatime behavior.

I think that's a different issue.

> The only time this changes is if you do mount -o remount,norelatime.

Hm, not on xfs:

# mount -o loop,norelatime xfsfile  mnt
# grep loop /proc/mounts
/dev/loop0 /tmp/mnt xfs rw,seclabel,relatime,attr2,inode64,logbufs=8,logbsize=32k,noquota 0 0

# mount -o remount,norelatime mnt
# grep loop /proc/mounts
/dev/loop0 /tmp/mnt xfs rw,seclabel,relatime,attr2,inode64,logbufs=8,logbsize=32k,noquota 0 0

Here, "norelatime" only makes the mount binary omit the MS_RELATIME flag.
The only way to override relatime behavior is mount -o strictatime, AFAICT.

IOWS "norelatime" and "strictatime" are the same (right?); perhaps
mount -o norelatime should set the MS_STRICTATIME flag.

> So my question is, what do we do here?  I know Christoph has the strong opinion that we just don't expose I_VERSION at all, which frankly I'm ok with.  However more what I'm asking is what do we do with these weird inverted flags that we all just kind of ignore on mount?  The current setup is just broken if we want to allow overriding the defaults at mount time.  Are we ok with it just being broken?  Are we ok with things like mount -o noiversion not working because the file system has decided that I_VERSION (or relatime) is the default, and we're going to ignore what the user asks for unless we're remounting?  Thanks,

Are there other oddities besides iversion and relatime?

-Eric