linux-kernel - Re: [PATCH v4 0/3] fs: allow statmount to fetch the fs_subtype and sb

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Message-ID: <20241113160037.74vu5yqzsfbvk3ad@quack3>
Date: Wed, 13 Nov 2024 17:00:37 +0100
From: Jan Kara <jack@...e.cz>
To: Miklos Szeredi <miklos@...redi.hu>
Cc: Jan Kara <jack@...e.cz>, Jeff Layton <jlayton@...nel.org>,
	Karel Zak <kzak@...hat.com>, Christian Brauner <brauner@...nel.org>,
	Ian Kent <raven@...maw.net>, Josef Bacik <josef@...icpanda.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Lennart Poettering <lennart@...ttering.net>
Subject: Re: [PATCH v4 0/3] fs: allow statmount to fetch the fs_subtype and
 sb_source

On Wed 13-11-24 16:49:52, Miklos Szeredi wrote:
> On Wed, 13 Nov 2024 at 16:18, Jan Kara <jack@...e.cz> wrote:
> >
> > On Wed 13-11-24 08:45:06, Jeff Layton wrote:
> > > On Wed, 2024-11-13 at 12:27 +0100, Karel Zak wrote:
> > > > On Tue, Nov 12, 2024 at 02:39:21PM GMT, Christian Brauner wrote:
> > > > Next on the wish list is a notification (a file descriptor that can be
> > > > used in epoll) that returns a 64-bit ID when there is a change in the
> > > > mount node. This will enable us to enhance systemd so that it does not
> > > > have to read the entire mount table after every change.
> > > >
> > >
> > > New fanotify events for mount table changes, perhaps?
> >
> > Now that I'm looking at it I'm not sure fanotify is a great fit for this
> > usecase. A lot of fanotify functionality does not really work for virtual
> > filesystems such as proc and hence we generally try to discourage use of
> > fanotify for them. So just supporting one type of event (like FAN_MODIFY)
> > on one file inside proc looks as rather inconsistent interface. But I
> > vaguely remember we were discussing some kind of mount event, weren't we?
> > Or was that for something else?
> 
> Yeah, if memory serves right what we agreed on was that placing a
> watch on a mount would result in events being generated for
> mount/umount/move_mount directly under that mount.  So this would not
> be monitoring the entire namespace as poll on /proc/$$/mountinfo does.
> IIRC Lennart said that this is okay and even desirable for systemd,
> since it's only interested in a particular set of mounts.

Oh, right. Thanks for reminding me. And yes, this would fit within what
fanotify supports quite nicely.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR