Re: [RFC 19/19] mm, netmem: remove the page pool members in struct page

[Matthew Wilcox]

On Fri, May 09, 2025 at 10:32:08AM -0700, Mina Almasry wrote:
> Currently the only restriction on net_iov is that some of its fields
> need to be cache aligned with some of the fields of struct page, but

Cache aligned?  Do you mean alias (ie be at the same offset)?

> What I would suggest here is, roughly:
> 
> 1. Add a new struct:
> 
>                struct netmem_desc {
>                        unsigned long pp_magic;
>                        struct page_pool *pp;
>                        unsigned long _pp_mapping_pad;
>                        unsigned long dma_addr;
>                        atomic_long_t pp_ref_count;
>                };
> 
> 2. Then update struct page to include this entry instead of the definitions:
> 
> struct page {
> ...
>                struct netmem_desc place_holder_1; /* for page pool */
> ...
> }

No, the point is to move these fields out of struct page entirely.

At some point (probably this year), we'll actually kmalloc the netmem_desc
(and shrink struct page), but for now, it'll overlap the other fields
in struct page.

> 3. And update struct net_iov to also include netmem_desc:
> 
> struct net_iov {
>     struct netmem_desc desc;
>     struct net_iov_area *owner;
>     /* More net_iov specific fields in the future */
> };
> 
> And drop patch 1 which does a rename.
> 
> Essentially netmem_desc can be an encapsulation of the shared fields
> between struct page and struct net_iov.

That is not the goal.