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Date: Fri, 7 Jun 2024 14:42:53 +0100
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Subject: Re: [PATCH net-next v10 02/14] net: page_pool: create hooks for
 custom page providers
To: Mina Almasry <almasrymina@google.com>
Cc: Christoph Hellwig <hch@infradead.org>, netdev@vger.kernel.org,
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References: <20240530201616.1316526-1-almasrymina@google.com>
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From: Pavel Begunkov <asml.silence@gmail.com>
In-Reply-To: <CAHS8izNmT_NzgCu1pY1RKgJh+kP2rCL_90Gqau2Pkd3-48Q1_w@mail.gmail.com>
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On 6/3/24 16:43, Mina Almasry wrote:
> On Mon, Jun 3, 2024 at 7:52 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
>>
>> On 6/3/24 15:17, Mina Almasry wrote:
>>> On Fri, May 31, 2024 at 10:35 PM Christoph Hellwig <hch@infradead.org> wrote:
>>>>
>>>> On Thu, May 30, 2024 at 08:16:01PM +0000, Mina Almasry wrote:
>>>>> I'm unsure if the discussion has been resolved yet. Sending the series
>>>>> anyway to get reviews/feedback on the (unrelated) rest of the series.
>>>>
>>>> As far as I'm concerned it is not.  I've not seen any convincing
>>>> argument for more than page/folio allocator including larger order /
>>>> huge page and dmabuf.
>>>>
>>>
>>> Thanks Christoph, this particular patch series adds dmabuf, so I
>>> assume no objection there. I assume the objection is that you want the
>>> generic, extensible hooks removed.
>>>
>>> To be honest, I don't think the hooks are an integral part of the
>>> design, and at this point I think we've argued for them enough. I
>>> think we can easily achieve the same thing with just raw if statements
>>> in a couple of places. We can always add the hooks if and only if we
>>> actually justify many memory providers.
>>>
>>> Any objections to me removing the hooks and directing to memory
>>> allocations via simple if statements? Something like (very rough
>>> draft, doesn't compile):
>>
>> The question for Christoph is what exactly is the objection here? Why we
>> would not be using well defined ops when we know there will be more
>> users? Repeating what I said in the last thread, for io_uring it's used
>> to implement the flow of buffers from userspace to the kernel, the ABI,
>> which is orthogonal to the issue of what memory type it is and how it
>> came there. And even if you mandate unnecessary dmabuf condoms for user
>> memory in one form or another IMHO for no clear reason, the callbacks
>> (or yet another if-else) would still be needed.
>>
>> Sure, Mina can drop and hard code devmem path to easy the pain for
>> him and delay the discussion, but then shortly after I will be
>> re-sending same shit.
> 
> You don't need to re-send the same ops again, right? You can add io
> uring support without ops. Something like:
> 
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index 92be1aaf18ccc..2cc986455bce6 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -557,8 +557,8 @@ netmem_ref page_pool_alloc_netmem(struct page_pool
> *pool, gfp_t gfp)
>                  return netmem;
> 
>          /* Slow-path: cache empty, do real allocation */
> -       if (static_branch_unlikely(&page_pool_mem_providers) && pool->mp_ops)
> -               netmem = pool->mp_ops->alloc_pages(pool, gfp);
> +       if (unlikely(page_pool_is_dmabuf(pool)))
> +               netmem = mp_dmabuf_devmem_alloc_pages():
> +       else if (unlikely(page_pool_is_iouring(pool)))
> +               netmem = mp_io_uring_alloc_pages():
>         else
>                  netmem = __page_pool_alloc_pages_slow(pool, gfp);
>          return netmem;
> 
> So IMO, the ops themselves, which Christoph is repeatedly nacking, are
> not that important.
> 
> I humbly think the energy should be spent convincing maintainers of
> the use case of io uring memory, not the ops. The ops are a cosmetic

I haven't seen any arguments against from the (net) maintainers so
far. Nor I see any objection against callbacks from them (considering
that either option adds an if).

And just not to confuse folks, it's just user pages, not some
weird special io_uring memory.

> change to the code, and can be added later. Christoph is nacking the
> ops because it gives people too much rope [1].

Yes, it is cosmetic, just as much as removing it is a cosmetic
change. You can apply same "too much rope" argument basically
to anything.

Take io_uring, nothing would change in the process, it'd still
be sent to net and reviewed exactly same way, while being less
clean, with poorer subsystem separation, allowing custom
formats / argument list, etc. I think it's cleaner with callbacks,
Mr. Christoph has other beliefs and keeps coercing to them,
even though from time to time it backfires for the author, just
personal experience.


> But if you disagree and think the ops themselves are important for a
> reason I missed, I'm happy waiting until agreement is reached here.
> Sorry, just voicing my 2 cents.
> 
> [1] https://lore.kernel.org/netdev/ZjjHUh1eINPg1wkn@infradead.org/
> 

-- 
Pavel Begunkov