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

Christoph Hellwig
2023-11-06T17:10:39Z

On Mon, Nov 06, 2023 at 02:05:45PM +0100, Christian Brauner wrote:
> > > I think spending time engaging this claim isn't worth it. This is just
> > > easily falsifiable via a simple grep for btrfs in systemd, lxc, runc,
> > > util-linux.
> >
> > Myabe you need to get our of your little bubble. There is plenty of
>
> Unnecessary personal comment, let alone that I'm not in any specific
> bubble just because I'm trying to be aware of what is currently going on
> in userspace.

Maybe you're just taking it to personal? A place where systemd, lxc,
runc, and util-linux are "all software" is a very much a bubble as you
won't find much userspace that stays more uptodate with particular
quirks of modern Linux features.

> Whatever you do here: vfsmounts or any other solution will force changes
> in userspace on a larger scale and changes to the filesystem itself. If
> you accommodate tar then you are fscking over other parts of userspace
> which are equally important. There is no free lunch.

It works for everything that knows that Linux mountpoint as exposed
in /proc/mounts and proc/self/mountinfo corresponds to the posix
definition of a mount point, and that one used on basically every
other unix system. It might not work as-is for software that actually
particularly knows how to manage btrfs subvolumes, but those are, by
defintion, not the problem anyway.

It's thinkgs like backup tools that run into random ino_t duplicates.
That's an example we had in the past, and I would be absolutely not be
surprised if there is more than more of those hiding right now.