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Date: Wed, 8 Nov 2023 22:55:52 -0800
From: Christoph Hellwig <hch@infradead.org>
To: Christian Brauner <brauner@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>,
	Josef Bacik <josef@toxicpanda.com>,
	Qu Wenruo <quwenruo.btrfs@gmx.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 Wed, Nov 08, 2023 at 05:20:06PM +0100, Christian Brauner wrote:
> This would also allow tools that want to to detect when they're crossing
> into a new subvolume - be it on btrfs or bcachefs - and take appropriate
> measures deciding what they want to do just relying on statx() without
> any additional system calls.

How?  If they want to only rely on Posix and not just he historical
unix/linux behavior they need to compare st_dev for the inode and it's
parent to see if it the Posix concept of a mount point (not to be
confused with the Linux concept of a mountpoint apparently) because
that allows the file system to use a new inode number namespace.