netdev - Re: [PATCH v3] Add BPF_SYNCHRONIZE_MAP_TO_MAP

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Date:   Mon, 30 Jul 2018 17:45:32 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Daniel Colascione <dancol@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>
Cc:     Joel Fernandes <joelaf@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Tim Murray <timmurray@...gle.com>,
        netdev <netdev@...r.kernel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Lorenzo Colitti <lorenzo@...gle.com>,
        Chenbo Feng <fengc@...gle.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Alexei Starovoitov <ast@...com>
Subject: Re: [PATCH v3] Add BPF_SYNCHRONIZE_MAP_TO_MAP_REFERENCES bpf(2)
 command

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@...earbox.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.

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...