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

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Message-ID: <20250714120617.GA36228@system.software.com>
Date: Mon, 14 Jul 2025 21:06:17 +0900
From: Byungchul Park <byungchul@...com>
To: Pavel Begunkov <asml.silence@...il.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 Mon, Jul 14, 2025 at 12:45:17PM +0100, Pavel Begunkov wrote:
> 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

Right.

> the rest, it might need to wait for the PGTY change, which is more

Only 3/8 needs to wait for the PGTY change.  The rest can be merged
regardless of it if acceptable.

	Byungchul

> likely to be for 6.18
> 
> --
> Pavel Begunkov