[PATCH net-next v18 07/14] memory-provider: dmabuf devmem memory provider

[Mina Almasry]

On Thu, Aug 8, 2024 at 10:24 PM Jakub Kicinski <kuba at kernel.org> wrote:
>
> On Thu, 8 Aug 2024 16:36:24 -0400 Mina Almasry wrote:
> > > How do you know that the driver:
> > >  - supports net_iov at all (let's not make implicit assumptions based
> > >    on presence of queue API);
> > >  - supports net_iov in current configuration (eg header-data split is
> > >    enabled)
> > >  - supports net_iov for _this_ pool (all drivers must have separate
> > >    buffer pools for headers and data for this to work, some will use
> > >    page pool for both)
> > >
> > > What comes to mind is adding an "I can gobble up net_iovs from this
> > > pool" flag in page pool params (the struct that comes from the driver),
> >
> > This already sorta exists in the current iteration, although maybe in
> > an implicit way. As written, drivers need to set params.queue,
> > otherwise core will not attempt to grab the mp information from
> > params.queue. A driver can set params.queue for its data pages pool
> > and not set it for the headers pool. AFAICT that deals with all 3
> > issues you present above.
> >
> > The awkward part is if params.queue starts getting used for other
> > reasons rather than passing mp configuration, but as of today that's
> > not the case so I didn't add the secondary flag. If you want a second
> > flag to be added preemptively, I can do that, no problem. Can you
> > confirm params.queue is not good enough?
>
> I'd prefer a flag. The setting queue in a param struct is not a good
> API for conveying that the page pool is for netmem payloads only.
>
> > > and then on the installation path we can check if after queue reset
> > > the refcount of the binding has increased. If it did - driver has
> > > created a pool as we expected, otherwise - fail, something must be off.
> > > Maybe that's a bit hacky?
> >
> > What's missing is for core to check at binding time that the driver
> > supports net_iov. I had relied on the implicit presence of the
> > queue-API.
> >
> > What you're proposing works, but AFAICT it's quite hacky, yes. I
> > basically need to ASSERT_RTNL in net_devmem_binding_get() to ensure
> > nothing can increment the refcount while the binding is happening so
> > that the refcount check is valid.
>
> True. Shooting from the hip, but we could walk the page pools of the
> netdev and find the one that has the right mp installed, and matches
> queue? The page pools are on a list hooked up to the netdev, trivial
> to walk.
>

I think this is good, and it doesn't seem hacky to me, because we can
check the page_pools of the netdev while we hold rtnl, so we can be
sure nothing is messing with the pp configuration in the meantime.
Like you say below it does validate the driver rather than rely on the
driver saying it's doing the right thing. I'll look into putting this
in the next version.


--
Thanks,
Mina