linux-kernel - Re: Upcoming: Notifications, FS notifications and fsinfo()

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Message-ID: <20200331081507.f6an4x32cxwpxdpd@wittgenstein>
Date:   Tue, 31 Mar 2020 10:15:07 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>, dray@...hat.com,
        Karel Zak <kzak@...hat.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Steven Whitehouse <swhiteho@...hat.com>,
        Jeff Layton <jlayton@...hat.com>, Ian Kent <raven@...maw.net>,
        andres@...razel.de, keyrings@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Lennart Poettering <lennart@...ttering.net>,
        Aleksa Sarai <cyphar@...har.com>
Subject: Re: Upcoming: Notifications, FS notifications and fsinfo()

On Tue, Mar 31, 2020 at 07:11:11AM +0200, Miklos Szeredi wrote:
> On Mon, Mar 30, 2020 at 11:17 PM Christian Brauner
> <christian.brauner@...ntu.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.

It at all that's only true for batch requests.

> 
> > 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

Yes. So have others, Al actively disliked and nacked it and no-one got
excited about it.

> issues or just vague bad feelings?

We have had that discussion on-list where I made my "vague bad feelings"
clear where you responded with the same dismissive style so I don't see
the point in repeating this experience.

Again, I want to make it clear that here I'm stating my preference as a
user of this api and as such I don't want to have to parse through a
filesystem to get complex information about filesystems. We've had
fruitful discussions [1] around how fsinfo() ties in with supervised
mounts and the rest of the mount api and its clear and simple especially
in the face of namespaces and implements a nice delegation model. So +1
from me.

Christian

[1]: https://youtu.be/LN2CUgp8deo?t=6840