Re: [PATCH net-next v7 1/7] netmem: introduce struct netmem_desc mirroring struct page

[Jakub Kicinski <kuba@kernel.org>]

On Fri, 27 Jun 2025 12:54:05 +0900 Byungchul Park wrote:
> On Thu, Jun 26, 2025 at 05:49:04PM -0700, Jakub Kicinski wrote:
> > On Wed, 25 Jun 2025 13:33:44 +0900 Byungchul Park wrote:  
> > > +/* A memory descriptor representing abstract networking I/O vectors,
> > > + * generally for non-pages memory that doesn't have its corresponding
> > > + * struct page and needs to be explicitly allocated through slab.  
> > 
> > I still don't get what your final object set is going to be.  
> 
> The ultimate goal is:
> 
>    Remove the pp fields from struct page
> 
> The second important goal is:
> 
>    Introduce a network pp descriptor, netmem_desc
> 
> While working on these two goals, I added some extra patches too, to
> clean up related code if it's obvious e.g. patches for renaming and so
> on.

Object set. Not objective.

> > We have
> >  - CPU-readable buffers (struct page)
> >  - un-readable buffers (struct net_iov)
> >  - abstract reference which can be a pointer to either of the
> >    above two (bitwise netmem_ref)
> > 
> > You say you want to evacuate page pool state from struct page
> > so I'd expect you to add a type which can always be fed into
> > some form of $type_to_virt(). A type which can always be cast
> > to net_iov, but not vice versa. So why are you putting things
> > inside net_iov, not outside.  
> 
> The type, struct netmem_desc, is declared outside.  Even though it's
> used overlaying on struct page *for now*, it will be dynamically
> allocated through slab shortly - it's also one of mm's plan.
> 
> As you know, net_iov is working with the assumption that it overlays on
> struct page *for now* indeed, when it comes to netmem_ref.  See the
> following APIs as example:
> 
> static inline struct net_iov *__netmem_clear_lsb(netmem_ref netmem)
> {
> 	return (struct net_iov *)((__force unsigned long)netmem & ~NET_IOV);
> }
> 
> static inline void netmem_set_pp(netmem_ref netmem, struct page_pool *pool)
> {
> 	__netmem_clear_lsb(netmem)->pp = pool;
> }
> 
> I'd say, I replaced the overlaying (on struct page) part with a
> well-defined struct, netmem_desc that will play the role of struct page
> for pp usage, instead of a set of the current overlaying fields of
> net_iov.
> 
> This 'introduction of netmem_desc' patch can be the base for network
> code to use netmem_desc as pp descriptor instead of struct page.  That's
> what I meant.
> 
> Am I missing something or got you wrong?  If yes, please explain in more
> detail then I will get back with the answer.

Ugh, you keep explaining the mechanics to me. Our goal here is not
just to move fields around and make it still compile :/

Let me ask you this way: you said "netmem_desc" will be allocated
thru slab "shortly". How will calling the equivalent of page_address()
on netmem_desc work at that stage? Feel free to refer me to the existing
docs if its covered..