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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Wed, 5 Aug 2020 09:43:38 +0200
Message-ID: <CAJfpegvxKTy+4Zk6banvxQ83PeFV7Xnt2Qv=kkOg57rxFKqVEg@mail.gmail.com>
Subject: Re: [PATCH 13/17] watch_queue: Implement mount topology and attribute
 change notifications [ver #5]
To:     Ian Kent <raven@themaw.net>
Cc:     David Howells <dhowells@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Casey Schaufler <casey@schaufler-ca.com>,
        Stephen Smalley <sds@tycho.nsa.gov>,
        Nicolas Dichtel <nicolas.dichtel@6wind.com>,
        Christian Brauner <christian@brauner.io>, andres@anarazel.de,
        Jeff Layton <jlayton@redhat.com>, dray@redhat.com,
        Karel Zak <kzak@redhat.com>, keyrings@vger.kernel.org,
        Linux API <linux-api@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org,
        LSM <linux-security-module@vger.kernel.org>,
        linux-kernel@vger.kernel.org
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On Wed, Aug 5, 2020 at 3:54 AM Ian Kent <raven@themaw.net> wrote:
>

> > > It's way more useful to have these in the notification than
> > > obtainable
> > > via fsinfo() IMHO.
> >
> > What is it useful for?
>
> Only to verify that you have seen all the notifications.
>
> If you have to grab that info with a separate call then the count
> isn't necessarily consistent because other notifications can occur
> while you grab it.

No, no no.   The watch queue will signal an overflow, without any
additional overhead for the normal case.  If you think of this as a
protocol stack, then the overflow detection happens on the transport
layer, instead of the application layer.  The application layer is
responsible for restoring state in case of a transport layer error,
but detection of that error is not the responsibility of the
application layer.


Thanks,
Miklos