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From:   Daniel Colascione <dancol@google.com>
Date:   Mon, 30 Jul 2018 03:25:43 -0700
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Subject: Re: [PATCH v3] Add BPF_SYNCHRONIZE_MAP_TO_MAP_REFERENCES bpf(2) command
To:     Daniel Borkmann <daniel@iogearbox.net>
Cc:     Joel Fernandes <joelaf@google.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Tim Murray <timmurray@google.com>,
        netdev <netdev@vger.kernel.org>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Lorenzo Colitti <lorenzo@google.com>,
        Chenbo Feng <fengc@google.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Alexei Starovoitov <ast@fb.com>
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On Mon, Jul 30, 2018 at 3:04 AM, Daniel Borkmann <daniel@iogearbox.net> wrote:
> Hmm, I don't think such UAPI as above is future-proof. In case we would want
> a similar mechanism in future for other maps, we would need a whole new bpf
> command or reuse BPF_SYNCHRONIZE_MAP_TO_MAP_REFERENCES as a workaround though
> the underlying map may not even be a map-to-map. Additionally, we don't have
> any map object at hand in the above, so we couldn't make any finer grained
> decisions either. Something like below would be more suitable and leaves room
> for extending this further in future.

YAGNI. Your proposed mechanism doesn't add anything under the current
implementation. It's also not clear how a map-specific synchronization
command is supposed to work in cases where we swap multiple map
references. Do we synchronize_rcu multiple times? Why would we impose
that inefficiency just for the sake of some non-specific future
extensibility? Add some kind of batching layer? The current approach
works for the anticipated use cases.

While my preference is not to talk about map-to-maps at all in the
user API and instead spec the thing as talking about map references in
general, I'd rather have something that talks about
references-to-maps-acquired-from-maps than this interface.