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

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Message-ID: <5ee839d6-2734-41c5-b34c-8d686c910bc8@gmail.com>
Date: Mon, 14 Jul 2025 10:43:35 +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 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.

> diff --git a/drivers/net/ethernet/intel/idpf/idpf_txrx.c b/drivers/net/ethernet/intel/idpf/idpf_txrx.c
> index cef9dfb877e8..adccc7c8e68f 100644
> --- a/drivers/net/ethernet/intel/idpf/idpf_txrx.c
> +++ b/drivers/net/ethernet/intel/idpf/idpf_txrx.c
> @@ -3266,7 +3266,7 @@ static u32 idpf_rx_hsplit_wa(const struct libeth_fqe *hdr,
>   			     struct libeth_fqe *buf, u32 data_len)
>   {
>   	u32 copy = data_len <= L1_CACHE_BYTES ? data_len : ETH_HLEN;
> -	struct page *hdr_page, *buf_page;
> +	struct netmem_desc *hdr_nmdesc, *buf_nmdesc;
>   	const void *src;
>   	void *dst;
>   
> @@ -3274,10 +3274,10 @@ static u32 idpf_rx_hsplit_wa(const struct libeth_fqe *hdr,
>   	    !libeth_rx_sync_for_cpu(buf, copy))
>   		return 0;
>   
> -	hdr_page = __netmem_to_page(hdr->netmem);
> -	buf_page = __netmem_to_page(buf->netmem);
> -	dst = page_address(hdr_page) + hdr->offset + hdr_page->pp->p.offset;
> -	src = page_address(buf_page) + buf->offset + buf_page->pp->p.offset;
> +	hdr_nmdesc = __netmem_to_nmdesc(hdr->netmem);
> +	buf_nmdesc = __netmem_to_nmdesc(buf->netmem);
> +	dst = nmdesc_address(hdr_nmdesc) + hdr->offset + hdr_nmdesc->pp->p.offset;
> +	src = nmdesc_address(buf_nmdesc) + buf->offset + buf_nmdesc->pp->p.offset;
>   
>   	memcpy(dst, src, LARGEST_ALIGN(copy));
>   	buf->offset += copy;
-- 
Pavel Begunkov