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

[Byungchul Park]

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.

> 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.

	Byungchul

> > + * net_iovs are allocated and used by networking code, and the size of
> > + * the chunk is PAGE_SIZE.
> 
> FWIW not for long. Patches to make the size of net_iov configurable
> are in progress.