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

[David Sterba <dsterba@suse.cz>]

On Thu, Nov 02, 2023 at 07:54:50AM +1030, 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.
> 
> And since we're here, also add the output when mounting a btrfs, to keep
> the dmesg paired.
> 
> Now mounting a btrfs with all default mkfs options would look like this:
> 
> [   81.906566] BTRFS info (device dm-8): mounting filesystem 633b5c16-afe3-4b79-b195-138fe145e4f2
> [   81.907494] BTRFS info (device dm-8): using crc32c (crc32c-intel) checksum algorithm
> [   81.908258] BTRFS info (device dm-8): using free space tree
> [   81.912644] BTRFS info (device dm-8): auto enabling async discard
> [   81.913277] BTRFS info (device dm-8): checking UUID tree
> [   91.668256] BTRFS info (device dm-8): unmounting filesystem 633b5c16-afe3-4b79-b195-138fe145e4f2

On a fresh look, I' suggest to write the messages like

BTRFS info (...): first mount of filesystem UUID
...
BTRFS info (...): last umount of filesystem UUID

This is not for each mount so it's slightly confusing.