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From:   Daniel Colascione <dancol@google.com>
Date:   Mon, 30 Jul 2018 17:50:15 -0700
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Subject: Re: [PATCH v3] Add BPF_SYNCHRONIZE_MAP_TO_MAP_REFERENCES bpf(2) command
To:     Jakub Kicinski <jakub.kicinski@netronome.com>
Cc:     Daniel Borkmann <daniel@iogearbox.net>,
        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 5:45 PM, Jakub Kicinski
<jakub.kicinski@netronome.com> wrote:
> On Mon, 30 Jul 2018 17:33:39 -0700, Daniel Colascione wrote:
>> On Mon, Jul 30, 2018 at 5:26 PM, Jakub Kicinski wrote:
>> > On Mon, 30 Jul 2018 03:25:43 -0700, Daniel Colascione wrote:
>> > > 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.
>> >
>> > FWIW in case of HW offload targeting a particular map may allow users
>> > to avoid a potentially slow sync with all the devices on the system.
>>
>> Sure. But such a thing doesn't exist right now (right?), and we can
>> add that more-efficient-in-that-one-case BPF interface when it lands.
>> I'd rather keep things simple for now.
>
> You mean map-in-map offload doesn't exist today?  True, but it's on the
> roadmap for Netronome.

Sounds cool. I'd still wait until map-in-map offload lands until
adding the map-specific API though.

> Tangentially it would be really useful for us to have a "the map has
> actually been freed" notification/barrier.  We have complaints of users
> creating huge maps and then trying to free and recreate them quickly,
> and the creation part failing with -ENOMEM, because the free from a
> workqueue didn't run, yet :(  It'd probably require a different API to
> solve than what's discussed here, but since we're talking about syncing
> things I thought I'd put it out there...

Yeah. What you're talking about is what I meant upthread when I
briefly mentioned a "refcount draining approach" --- you'd block until
all references except your own to a map disappeared.