Re: [PATCH 01/18] netmem: introduce struct netmem_desc struct_group_tagged()'ed on struct net_iov

Toke Høiland-Jørgensen
2025-05-23T09:01:07Z

Byungchul Park <byungchul@sk.com> writes:

> To simplify struct page, the page pool members of struct page should be
> moved to other, allowing these members to be removed from struct page.
>
> Introduce a network memory descriptor to store the members, struct
> netmem_desc, reusing struct net_iov that already mirrored struct page.
>
> While at it, relocate _pp_mapping_pad to group struct net_iov's fields.
>
> Signed-off-by: Byungchul Park <byungchul@sk.com>
> ---
> include/linux/mm_types.h | 2 +-
> include/net/netmem.h | 43 +++++++++++++++++++++++++++++++++-------
> 2 files changed, 37 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 56d07edd01f9..873e820e1521 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -120,13 +120,13 @@ struct page {
> unsigned long private;
> };
> struct { /* page_pool used by netstack */
> + unsigned long _pp_mapping_pad;
> /**
> * @pp_magic: magic value to avoid recycling non
> * page_pool allocated pages.
> */
> unsigned long pp_magic;
> struct page_pool *pp;
> - unsigned long _pp_mapping_pad;
> unsigned long dma_addr;
> atomic_long_t pp_ref_count;
> };

The reason that field is called "_pp_mapping_pad" is that it's supposed
to overlay the page->mapping field, so that none of the page_pool uses
set a value here. Moving it breaks that assumption. Once struct
netmem_desc is completely decoupled from struct page this obviously
doesn't matter, but I think it does today? At least, trying to use that
field for the DMA index broke things, which is why we ended up with the
bit-stuffing in pp_magic...

-Toke