Re: [PATCH] btrfs: indicate iversion option in show_options

[Eric Sandeen <sandeen@redhat.com>]

On 7/29/20 9:46 AM, Josef Bacik wrote:
> Eric reported a problem where if you did
> 
> mount -o remount /some/btrfs/fs
> 
> you would lose SB_I_VERSION on the mountpoint.  After a very convoluted
> search I discovered this is because the remount infrastructure doesn't
> just say "change these things specifically", but it actually depends on
> userspace to tell it fucking everything that needs to be set on the
> mountpoint.  This led to the fucking horrifying discovery that
> util-linux actually has to parse /proc/mounts to figure out what the
> fuck is set on the mount point in order to preserve any of the options
> it's not actually fucking with, so in this case iversion.  If we don't
> indicate iversion is set, then we get iversion cleared on the mount,
> because util-linux doesn't pass in MS_I_VERSION as it's mount flags.
> 
> So work around this fucking insanity by spitting out iversion in
> /proc/mounts so we get the correct flags passed to us in remount.

Hmmm:

# mount -o loop,noiversion btrfsfile mnt
# grep btrfs /proc/mounts
/dev/loop0 /tmp/mnt btrfs rw,seclabel,relatime,iversion,space_cache,subvolid=5,subvol=/ 0 0
#

> Reported-by: Eric Sandeen <sandeen@redhat.com>
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  fs/btrfs/super.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index aa73422b0678..fe64aa2f5c7a 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -1427,6 +1427,10 @@ static int btrfs_show_options(struct seq_file *seq, struct dentry *dentry)
>  		seq_puts(seq, ",discard=async");
>  	if (!(info->sb->s_flags & SB_POSIXACL))
>  		seq_puts(seq, ",noacl");
> +	if (info->sb->s_flags & SB_I_VERSION)
> +		seq_puts(seq, ",iversion");
> +	else
> +		seq_puts(seq, ",noiversion");
>  	if (btrfs_test_opt(info, SPACE_CACHE))
>  		seq_puts(seq, ",space_cache");
>  	else if (btrfs_test_opt(info, FREE_SPACE_TREE))
>