Re: [PATCH] btrfs: add dmesg output when mounting and unmounting

[David Sterba <dsterba@suse.cz>]

On Thu, Nov 02, 2023 at 09:16:19AM +0800, Anand Jain wrote:
> On 11/2/23 05:24, Qu Wenruo wrote:
> > There is a feature request to add dmesg output when unmounting a btrfs.
> > 
> > There are several alternative methods to do the same thing, but with
> > their problems:
> > 
> > - Use eBPF to watch btrfs_put_super()/open_ctree()
> >    Not end user friendly, they have to dip their head into the source
> >    code.
> > 
> > - Watch for /sys/fs/<uuid>/
> >    This is way more simpler, but still requires some simple device -> uuid
> >    lookups.
> >    And a script needs to use inotify to watch /sys/fs/.
> > 
> 
> 
> > Compared to all these, directly outputting the information into dmesg
> > would be the most simple one, with both device and UUID included.
> > 
> 
> Well, I submitted a patch for this in 2017, but I'm not sure why it 
> wasn't integrated or commented.
> 
> https://lore.kernel.org/linux-btrfs/3352043d-dbb1-0055-f50a-c91ca43aff1d@oracle.com/

I think you sent it more than once and that I replied, messages about
mounting each subvolume (as in your patch) seem to be duplicating what
eg. audit or some policy layer should do. The same comment is in the
github issue.

The patch here and the request is to track only the first and last mount
of the same filesystem with a bit different use case, eg. properly
waiting until lazy umount finishe, or in container environments to be
sure that the last mount is gone.

The suggested waiting on /sys/fs/btrfs/UUID could eventually wrapped in
a subcommand, for conveniencie.