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

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Message-ID: <20250713233934.GA26068@system.software.com>
Date: Mon, 14 Jul 2025 08:39:34 +0900
From: Byungchul Park <byungchul@...com>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: willy@...radead.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	kernel_team@...ynix.com, kuba@...nel.org, almasrymina@...gle.com,
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	jackmanb@...gle.com
Subject: Re: [PATCH net-next v9 2/8] netmem: introduce utility APIs to use
 struct netmem_desc

On Mon, Jul 14, 2025 at 08:07:52AM +0900, 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.

As Mina suggested, I will add this helper along with the patch using it.
	Byungchul

> 	Byungchul
> 
> ---8<---
> 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;
> --
> > > +
> > >   /**
> > >    * __netmem_address - unsafely get pointer to the memory backing @netmem
> > >    * @netmem: netmem reference to get the pointer for
> > 
> > --
> > Pavel Begunkov