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Date: Wed, 8 Nov 2023 06:07:10 -0800
From: Christoph Hellwig <hch@infradead.org>
To: Christian Brauner <brauner@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>,
	Qu Wenruo <quwenruo.btrfs@gmx.com>,
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	Chris Mason <clm@fb.com>, David Sterba <dsterba@suse.com>,
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Subject: Re: [PATCH 0/3] fanotify support for btrfs sub-volumes
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On Wed, Nov 08, 2023 at 09:22:33AM +0100, Christian Brauner wrote:
> > 	/mnt/1/foo to /mnt/2/bar will get your dev 8 for /mnt/2/bar
> > 
> > So the device number changes at the mount point here, bind mount or not.
> 
> Yes, I know. But /mnt/2/ will have the device number of the
> superblock/filesystem it belongs to and so will /mnt/1. Creating a
> bind-mount won't suddenly change the device number and decoupling it
> from the superblock it is a bind-mount of.

It doesn't any more then just changing st_dev.  But at least it aligns
to the boundary that such a change always aligned to in not just Linux
but most (if not all?) Unix variants and thus where it is expected.