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

From: Christoph Hellwig <hch@infradead.org>
To: Christian Brauner <brauner@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>,
	Qu Wenruo <quwenruo.btrfs@gmx.com>,
	Josef Bacik <josef@toxicpanda.com>,
	Amir Goldstein <amir73il@gmail.com>, Jan Kara <jack@suse.cz>,
	Chris Mason <clm@fb.com>, David Sterba <dsterba@suse.com>,
	linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 0/3] fanotify support for btrfs sub-volumes
Date: Mon, 6 Nov 2023 09:10:28 -0800	[thread overview]
Message-ID: <ZUkeBM1sik1daE1N@infradead.org> (raw)
In-Reply-To: <20231106-datei-filzstift-c62abf899f8f@brauner>

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.