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

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Message-ID: <0b154b9b-728f-7d57-d4c5-ec25fc9dfdf3@toxicpanda.com>
Date:   Wed, 29 Jul 2020 14:32:30 -0400
From:   Josef Bacik <josef@...icpanda.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Eric Sandeen <sandeen@...hat.com>,
        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: Inverted mount options completely broken (iversion,relatime)

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

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,

Josef