Re: [PATCH 0/3] fanotify support for btrfs sub-volumes

[Christoph Hellwig <hch@infradead.org>]

On Tue, Oct 31, 2023 at 01:14:42PM +0100, Christian Brauner wrote:
> What happens in the kernel right now I've mentiond in the mount api
> conversion patch for btrfs I sent out in June at [1] because I tweaked
> that behavior. Say I mount both subvolumes:
> 
> mount /dev/sda -o subvol=subvol1 /vol1 # sb1@vfsmount1
> mount /dev/sda -o subvol=subvol2 /vol2 # sb1@vfsmount2
> 
> It creates a superblock for /dev/sda. It then creates two vfsmounts: one
> for subvol1 and one for subvol2. So you end up with two subvolumes on
> the same superblock.
> 
> So if you mount a subvolume today then you already get separate
> vfsmounts. To put it another way. If you start 10,000 containers each
> using a separate btrfs subvolume then you get 10,000 vfsmounts.

But only if you mount them explicitly, which you don't have to.

> Or is it that you want a separate superblock per subvolume?

Does "you" refer to me here?  No, I don't.

> Because only
> if you allocate a new superblock you'll get clean device number
> handling, no? Or am I misunderstanding this?

If you allocate a super block you get it for free.  If you don't
you have to manually allocate it report it in ->getattr.