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

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Message-ID: <CAHS8izOXwZS-8sfvn3DuT1XWhjc--7-ZLjr8rMn1XHr5F+ckbA@mail.gmail.com>
Date: Sat, 10 Aug 2024 22:21:39 -0400
From: Mina Almasry <almasrymina@...gle.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Pavel Begunkov <asml.silence@...il.com>, netdev@...r.kernel.org, 
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	Kaiyuan Zhang <kaiyuanz@...gle.com>
Subject: Re: [PATCH net-next v18 07/14] memory-provider: dmabuf devmem memory provider

On Fri, Aug 9, 2024 at 11:52 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Fri, 9 Aug 2024 16:45:50 +0100 Pavel Begunkov wrote:
> > > 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.
> >
> > Why not have a flag set by the driver and advertising whether it
> > supports providers or not, which should be checked for instance in
> > netdev_rx_queue_restart()? If set, the driver should do the right
> > thing. That's in addition to a new pp_params flag explicitly telling
> > if pp should use providers. It's more explicit and feels a little
> > less hacky.
>
> You mean like I suggested in the previous two emails? :)
>
> Given how easy the check is to implement, I think it's worth
> adding as a sanity check. But the flag should be the main API,
> if the sanity check starts to be annoying we'll ditch it.

I think we're talking about 2 slightly different flags, AFAIU.

Pavel and I are suggesting the driver reports "I support memory
providers" directly to core (via the queue-api or what not), and we
check that flag directly in netdev_rx_queue_restart(), and fail
immediately if the support is not there.

Jakub is suggesting a page_pool_params flag which lets the driver
report "I support memory providers". If the driver doesn't support it
but core is trying to configure that, then the page_pool_create will
fail, which will cause the queue API operation
(ndo_queue_alloc_mem_alloc) to fail, which causes
netdev_rx_queue_restart() to fail.

Both are fine, I don't see any extremely strong reason to pick one of
the other. I prefer Jakub's suggestion, just because it's closer to
the page_pool and may be more reusable in the future. I'll err on the
side of that unless I hear strong preference to the contrary.

I also think the additional check that Jakub is requesting is easy to
implement and unobjectionable. It would let core validate that the
driver did actually create the page_pool with the memory provider. I
think one of the goals of the queue API was to allow core to do more
validation on driver configuration anyway.

-- 
Thanks,
Mina