netdev - Re: [PATCH net-next v9 2/8] netmem: introduce utility APIs to use struct netmem

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Message-ID: <ac59fba9-4f39-4691-afae-aa7a0b1270af@gmail.com>
Date: Mon, 14 Jul 2025 12:45:17 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Byungchul Park <byungchul@...com>
Cc: willy@...radead.org, netdev@...r.kernel.org,
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Subject: Re: [PATCH net-next v9 2/8] netmem: introduce utility APIs to use
 struct netmem_desc

On 7/14/25 11:05, Byungchul Park wrote:
> On Mon, Jul 14, 2025 at 10:43:35AM +0100, Pavel Begunkov wrote:
>> On 7/14/25 00:07, Byungchul Park wrote:
>>> On Sat, Jul 12, 2025 at 12:59:34PM +0100, Pavel Begunkov wrote:
>>>> On 7/10/25 09:28, Byungchul Park wrote:
>>>> ...> +
>>>>>     static inline struct net_iov *netmem_to_net_iov(netmem_ref netmem)
>>>>>     {
>>>>>         if (netmem_is_net_iov(netmem))
>>>>> @@ -314,6 +340,21 @@ static inline netmem_ref netmem_compound_head(netmem_ref netmem)
>>>>>         return page_to_netmem(compound_head(netmem_to_page(netmem)));
>>>>>     }
>>>>>
>>>>> +#define nmdesc_to_page(nmdesc)               (_Generic((nmdesc),             \
>>>>> +     const struct netmem_desc * :    (const struct page *)(nmdesc),  \
>>>>> +     struct netmem_desc * :          (struct page *)(nmdesc)))
>>>>
>>>> Considering that nmdesc is going to be separated from pages and
>>>> accessed through indirection, and back reference to the page is
>>>> not needed (at least for net/), this helper shouldn't even exist.
>>>> And in fact, you don't really use it ...
>>>>> +static inline struct netmem_desc *page_to_nmdesc(struct page *page)
>>>>> +{
>>>>> +     VM_BUG_ON_PAGE(PageTail(page), page);
>>>>> +     return (struct netmem_desc *)page;
>>>>> +}
>>>>> +
>>>>> +static inline void *nmdesc_address(struct netmem_desc *nmdesc)
>>>>> +{
>>>>> +     return page_address(nmdesc_to_page(nmdesc));
>>>>> +}
>>>>
>>>> ... That's the only caller, and nmdesc_address() is not used, so
>>>> just nuke both of them. This helper doesn't even make sense.
>>>>
>>>> Please avoid introducing functions that you don't use as a general
>>>> rule.
>>>
>>> I'm sorry about making you confused.  I should've included another patch
>>> using the helper like the following.
>>
>> Ah, I see. And still, it's not a great function. There should be
>> no way to extract a page or a page address from a nmdesc.
>>
>> For the diff below it's same as with the mt76 patch, it's allocating
>> a page, expects it to be a page, using it as a page, but for no reason
>> keeps it wrapped into netmem. It only adds confusion and overhead.
>> A rule of thumb would be only converting to netmem if the new code
>> would be able to work with a netmem-wrapped net_iovs.
> 
> Thanks.  I'm now working on this job, avoiding your concern.
> 
> By the way, am I supposed to wait for you to complete the work about
> extracting type from page e.g. page pool (or bump) type?

1/8 doesn't depend on it, if you're sending it separately. As for
the rest, it might need to wait for the PGTY change, which is more
likely to be for 6.18

-- 
Pavel Begunkov