Message-ID: <7e2ffe62-032a-4c5e-953b-b7117ab076be@gmail.com>
Date: Mon, 12 Aug 2024 19:57:51 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
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Subject: Re: [PATCH net-next v18 07/14] memory-provider: dmabuf devmem memory
 provider

On 8/12/24 18:57, Jakub Kicinski wrote:
> On Sun, 11 Aug 2024 22:51:13 +0100 Pavel Begunkov wrote:
>>> 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.
>>
>> I might've misread Jakub, but yes, I believe it's different. It'd
>> communicate about support for providers to upper layers, so we can
>> fail even before attempting to allocate a new queue and init a
>> page pool.
> 
> Got it. Since allocating memory happens before stopping traffic
> I think it's acceptable to stick to a single flag.

I agree, it's an failure case of init path, shouldn't be
a problem.


>>> 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.
>>
>> And I'm not against this way either if we explicitly get an error
>> back instead of trying to figure it out post-factum like by
>> checking the references and possibly reverting the allocation.
>> Maybe that's where I was confused, and that refcount thing was
>> suggested as a WARN_ONCE?
> 
> Yup, the refcount (now: check of the page pool list) was meant
> as a WARN_ONCE() to catch bad drivers.
> 
>> FWIW, I think it warrants two flags. The first saying that the
>> driver supports providers at all:
>>
>> page_pool_init() {
>> 	if (rxq->mp_params)
>> 		if (!(flags & PP_PROVIDERS_SUPPORTED))
>> 			goto fail;
>> }
>>
>> And the second telling whether the driver wants to install
>> providers for this particular page pool, so if there is a
>> separate pool for headers we can set it with plain old kernel
>> pages.
> 
> The implementation of the queue API should be resilient against
> failures in alloc, and not being MP capable is just a form of
> alloc failure. I don't see the upside of double-flag.
> 
>> payload_pool = page_pool_create(rqx, PP_PROVIDERS_SUPPORTED);
>> header_pool = page_pool_create(rqx, PP_PROVIDERS_SUPPORTED |
>>                                       PP_IGNORE_PROVIDERS);
> 
> Also don't see the upside of the explicit "non-capable" flag,
> but I haven't thought of that. Is there any use?

Considering that we pass a mp to page pool indirectly via
a queue

rxq->mp_param = devmemtcp;
... // later
pp_params.queue = rxq;
page_pool_create(pp_params);

How can we create a separate header pool without mp and let
the other data pool be initialized with mp? We can make
sure pp_params.queue is NULL, but API isn't sound, I think
the plans are to reuse the queue argument for some other
purposes.

param_tmp = rxq->mp_param;
rxq->mp_param = NULL;
page_pool_create(pp_params);
rxq->mp_param = param_tmp;

Temporarily erasing mp_param is another option, but I think
by far the simplest is another flag.

> One important note. The flag should not be tied to memory providers
> but rather to netmem, IOW unreadable memory. MP is an internal detail,
> the important fact from the driver-facing API perspective is that the
> driver doesn't need struct pages.

Sure, didn't want to go into describing how these are not
the same thing. Naming should reflect it, and if we expect
providers that don't produce net_iov, we might want to have
a flag in mp_param or so.

-- 
Pavel Begunkov

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