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Date: Wed, 8 Nov 2023 06:08:09 -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>,
	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
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On Wed, Nov 08, 2023 at 09:27:44AM +0100, Christian Brauner wrote:
> > What is that flag going to buy us?
> 
> The initial list that Josef provided in
> https://lore.kernel.org/linux-btrfs/20231025210654.GA2892534@perftesting
> asks to give users a way to figure out whether a file is located on a
> subvolume. Which I think is reasonable and there's a good chunk of
> software out there that could really benefit from this. Now all of the
> additional info that is requested doesn't need to live in statx(). But
> that flag can serve as an indicator for userspace that they are on a
> subvolume and that they can go to btrfs specific ioctls if they want to
> figure out more details.

Well, if we want to legitimize the historic btrfs behavior the way to
find out is if st_dev changes without that being a mount point, so
an extra flag would be redundant.