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

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2023/11/3 06:48, David Sterba wrote:
> 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.

My bad, I didn't realize until I crafted the `btrfs monitor watch` command.

Inotify just doesn't work on sysfs....

So I believe for now, the patch is the best way to go, unless we want to
something like checking the sysfs every second...

Thanks,
Qu