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Date:   Tue, 31 Oct 2023 10:06:17 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Christian Brauner <brauner@kernel.org>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Josef Bacik <josef@toxicpanda.com>,
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        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
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On Tue, Oct 31, 2023 at 01:50:46PM +0100, Christian Brauner wrote:
> So this is effectively a request for:
> 
> btrfs subvolume create /mnt/subvol1
> 
> to create vfsmounts? IOW,
> 
> mkfs.btrfs /dev/sda
> mount /dev/sda /mnt
> btrfs subvolume create /mnt/subvol1
> btrfs subvolume create /mnt/subvol2
> 
> would create two new vfsmounts that are exposed in /proc/<pid>/mountinfo
> afterwards?

Yes.

> That might be odd. Because these vfsmounts aren't really mounted, no?

Why aren't they?

> And so you'd be showing potentially hundreds of mounts in
> /proc/<pid>/mountinfo that you can't unmount?

Why would you not allow them to be unmounted?

> And even if you treat them as mounted what would unmounting mean?

The code in btrfs_lookup_dentry that does a hand crafted version
of the file system / subvolume crossing (the location.type !=
BTRFS_INODE_ITEM_KEY one) would not be executed.