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

[Byungchul Park]

On Tue, Jul 01, 2025 at 04:45:08PM -0700, Jakub Kicinski wrote:
> On Mon, 30 Jun 2025 08:34:48 +0900 Harry Yoo wrote:
> > > 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..
> >
> > https://kernelnewbies.org/MatthewWilcox/Memdescs/Path
> > https://kernelnewbies.org/MatthewWilcox/Memdescs
> >
> > May not be the exact document you're looking for,
> > but with this article I can imagine:
> >
> > - The ultimate goal is to shrink struct page to eventually from 64 bytes
> >   to 8 bytes, by allocating only the minimum required metadata per 4k page
> >   statically and moving the rest of metadata to dynamically-allocated
> >   descriptors (netmem_desc, anon, file, ptdesc, zpdesc, etc.) using slab
> >   at page allocation time.
> >
> > - We can't achieve that goal just yet, because several subsystems
> >   still use struct page fields for their own purposes.
> >
> >   To achieve that, each of these subsystems needs to define
> >   its own descriptor, which, for now, overlays struct page, and should be
> >   converted to use the new descriptor.
> >
> >   Eventually, these descriptors will be allocated using slab.
> >
> > - For CPU-readable buffers, page->memdesc will point to a netmem_desc,
> >   with a lower bit set indicating that it's a netmem_desc rather than
> >   other type. Networking code will need to cast it to (netmem_desc *)
> >   and dereference it to access networking specific fields.
> >
> > - The struct page array (vmemmap) will still be statically allocated
> >   at boot time (or during memory hotplug time).
> >   So no change in how page_address() works.
> >
> > net_iovs will continue to be not associated with struct pages,
> > as the buffers don't have corresponding struct pages.
> > net_iovs are already allocated using slab.
> 
> Thanks a lot, this clarifies things for me.
> 
> Unfortunately, I still think that it's hard to judge patches 1 and 7
> in context limited to this series, so let's proceed to reposting just
> the "middle 5" patches.

Just in case, I sent v8 with the "middle 5" last week as you requested.
I'm convinced they are non-controversial but lemme know if any.

	Byungchul