netdev - Re: [PATCH net-next v10 02/12] netmem: use netmem_desc instead of page to access ->pp in __netmem_get

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Message-ID: <9bed2f6e-6251-4d0c-ad1e-f1b8625a0a10@gmail.com>
Date: Tue, 15 Jul 2025 11:37:45 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Mina Almasry <almasrymina@...gle.com>, Byungchul Park <byungchul@...com>
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Subject: Re: [PATCH net-next v10 02/12] netmem: use netmem_desc instead of
 page to access ->pp in __netmem_get_pp()

On 7/14/25 20:37, Mina Almasry wrote:
> On Mon, Jul 14, 2025 at 5:01 AM Byungchul Park <byungchul@...com> wrote:
...>> +static inline struct netmem_desc *pp_page_to_nmdesc(struct page *page)
>> +{
>> +       DEBUG_NET_WARN_ON_ONCE(!page_pool_page_is_pp(page));
>> +
>> +       /* XXX: How to extract netmem_desc from page must be changed,
>> +        * once netmem_desc no longer overlays on page and will be
>> +        * allocated through slab.
>> +        */
>> +       return (struct netmem_desc *)page;
>> +}
>> +
> 
> Same thing. Do not create a generic looking pp_page_to_nmdesc helper
> which does not check that the page is the correct type. The
> DEBUG_NET... is not good enough.
> 
> You don't need to add a generic helper here. There is only one call
> site. Open code this in the callsite. The one callsite is marked as
> unsafe, only called by code that knows that the netmem is specifically
> a pp page. Open code this in the unsafe callsite, instead of creating
> a generic looking unsafe helper and not even documenting it's unsafe.
> 
>>   /**
>>    * __netmem_get_pp - unsafely get pointer to the &page_pool backing @netmem
>>    * @netmem: netmem reference to get the pointer from
>> @@ -280,7 +291,7 @@ static inline struct net_iov *__netmem_clear_lsb(netmem_ref netmem)
>>    */
>>   static inline struct page_pool *__netmem_get_pp(netmem_ref netmem)
>>   {
>> -       return __netmem_to_page(netmem)->pp;
>> +       return pp_page_to_nmdesc(__netmem_to_page(netmem))->pp;
>>   }
> 
> This makes me very sad. Casting from netmem -> page -> nmdesc...

The function is not used, and I don't think the series adds any
new users? It can be killed then. It's a horrible function anyway,
would be much better to have a variant taking struct page * if
necessary.

> Instead, we should be able to go from netmem directly to nmdesc. I
> would suggest rename __netmem_clear_lsb to netmem_to_nmdesc and have
> it return netmem_desc instead of net_iov. Then use it here.

Glad you liked the diff I suggested :) In either case, seems
like it's not strictly necessary for this iteration as
__netmem_get_pp() should be killed, and the rest of patches work
directly with pages.

  
> We could have an unsafe version of netmem_to_nmdesc which converts the
> netmem to netmem_desc without clearing the lsb and mark it unsafe.
> 

-- 
Pavel Begunkov