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

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Date:   Wed, 29 Jul 2020 14:47:43 -0400
From:   Josef Bacik <josef@...icpanda.com>
To:     Eric Sandeen <sandeen@...hat.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 2:41 PM, Eric Sandeen wrote:
> 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
> 

Oops you're right, I'm blind.  Same happens for btrfs, so using -o norelatime 
simply does nothing because it's considered a kernel wide default.

> 
> Are there other oddities besides iversion and relatime?

It doesn't look like it, I checked a few others of the MS_INVERT variety, these 
appear to be the only ones.  I really don't want to have this discussion again 
in the future tho when we introduce MS_SOME_NEW_AWESOME.  Thanks,

Josef