linux-kernel - Re: [RFC][PATCH 0/7] Mount, FS, Block and Keyrings notifications

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Date:   Thu, 30 May 2019 13:00:24 +0200
From:   Jan Kara <jack@...e.cz>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     Jan Kara <jack@...e.cz>, David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>, Ian Kent <raven@...maw.net>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-api@...r.kernel.org,
        linux-block <linux-block@...r.kernel.org>,
        keyrings@...r.kernel.org,
        LSM List <linux-security-module@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 0/7] Mount, FS, Block and Keyrings notifications

On Wed 29-05-19 18:53:21, Amir Goldstein wrote:
> > > David,
> > >
> > > I am interested to know how you envision filesystem notifications would
> > > look with this interface.
> > >
> > > fanotify can certainly benefit from providing a ring buffer interface to read
> > > events.
> > >
> > > From what I have seen, a common practice of users is to monitor mounts
> > > (somehow) and place FAN_MARK_MOUNT fanotify watches dynamically.
> > > It'd be good if those users can use a single watch mechanism/API for
> > > watching the mount namespace and filesystem events within mounts.
> > >
> > > A similar usability concern is with sb_notify and FAN_MARK_FILESYSTEM.
> > > It provides users with two complete different mechanisms to watch error
> > > and filesystem events. That is generally not a good thing to have.
> > >
> > > I am not asking that you implement fs_notify() before merging sb_notify()
> > > and I understand that you have a use case for sb_notify().
> > > I am asking that you show me the path towards a unified API (how a
> > > typical program would look like), so that we know before merging your
> > > new API that it could be extended to accommodate fsnotify events
> > > where the final result will look wholesome to users.
> >
> > Are you sure we want to combine notification about file changes etc. with
> > administrator-type notifications about the filesystem? To me these two
> > sound like rather different (although sometimes related) things.
> >
> 
> Well I am sure that ring buffer for fanotify events would be useful, so
> seeing that David is proposing a generic notification mechanism, I wanted
> to know how that mechanism could best share infrastructure with fsnotify.
> 
> But apart from that I foresee the questions from users about why the
> mount notification API and filesystem events API do not have better
> integration.
> 
> The way I see it, the notification queue can serve several classes
> of notifications and fsnotify could be one of those classes
> (at least FAN_CLASS_NOTIF fits nicely to the model).

I agree that for some type of fsnotify uses a ring buffer would make sense.
But for others - such as permission events or unlimited queues - you cannot
really use the ring buffer and I don't like the idea of having different
ways of passing fsnotify events to userspace based on notification group
type...

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