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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Tue, 31 Mar 2020 07:11:11 +0200
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Subject: Re: Upcoming: Notifications, FS notifications and fsinfo()
To:     Christian Brauner <christian.brauner@ubuntu.com>
Cc:     David Howells <dhowells@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>, dray@redhat.com,
        Karel Zak <kzak@redhat.com>,
        Miklos Szeredi <mszeredi@redhat.com>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Jeff Layton <jlayton@redhat.com>, Ian Kent <raven@themaw.net>,
        andres@anarazel.de, keyrings@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Lennart Poettering <lennart@poettering.net>,
        Aleksa Sarai <cyphar@cyphar.com>
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On Mon, Mar 30, 2020 at 11:17 PM Christian Brauner
<christian.brauner@ubuntu.com> wrote:

> Fwiw, putting down my kernel hat and speaking as someone who maintains
> two container runtimes and various other low-level bits and pieces in
> userspace who'd make heavy use of this stuff I would prefer the fd-based
> fsinfo() approach especially in the light of across namespace
> operations, querying all properties of a mount atomically all-at-once,

fsinfo(2) doesn't meet the atomically all-at-once requirement.  Sure,
it's possible to check the various change counters before and after a
batch of calls to check that the result is consistent.  Still, that's
not an atomic all-at-once query, if you'd really require that, than
fsinfo(2) as it currently stands would be inadequate.

> and safe delegation through fds. Another heavy user of this would be
> systemd (Cced Lennart who I've discussed this with) which would prefer
> the fd-based approach as well. I think pulling this into a filesystem
> and making userspace parse around in a filesystem tree to query mount
> information is the wrong approach and will get messy pretty quickly
> especially in the face of mount and user namespace interactions and
> various other pitfalls.

Have you actually looked at my proposed patch?   Do you have concrete
issues or just vague bad feelings?

Thanks,
Miklos