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

["Toke Høiland-Jørgensen" <toke@redhat.com>]

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