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

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Message-ID: <20250714100551.GA44803@system.software.com>
Date: Mon, 14 Jul 2025 19:05:51 +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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Subject: Re: [PATCH net-next v9 2/8] netmem: introduce utility APIs to use
 struct netmem_desc

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?

	Byungchul

> > 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