Re: [RFC v3 01/18] netmem: introduce struct netmem_desc mirroring struct page

Mina Almasry
2025-05-29T16:31:54Z

On Wed, May 28, 2025 at 8:11 PM Byungchul Park <byungchul@sk.com> wrote:
>
> 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 and make it union'ed with the existing fields in struct
> net_iov, allowing to organize the fields of struct net_iov. The final
> look of struct net_iov should be like:
>
> struct net_iov {
> struct netmem_desc;
> net_field1; /* e.g. struct net_iov_area *owner; */
> net_field2;
> ...
> };
>
> Signed-off-by: Byungchul Park <byungchul@sk.com>

This version looks much better from networking POV, and I think with
small adjustments we can make it work I think. I'll leave Matthew to
confirm if it's aligned with the memdesc plans.

> ---
> include/net/netmem.h | 101 +++++++++++++++++++++++++++++++++----------
> 1 file changed, 79 insertions(+), 22 deletions(-)
>
> diff --git a/include/net/netmem.h b/include/net/netmem.h
> index 386164fb9c18..d52f86082271 100644
> --- a/include/net/netmem.h
> +++ b/include/net/netmem.h
> @@ -12,6 +12,47 @@
> #include <linux/mm.h>
> #include <net/net_debug.h>
>
> +/* These fields in struct page are used by the page_pool and net stack:
> + *
> + * struct {
> + * unsigned long pp_magic;
> + * struct page_pool *pp;
> + * unsigned long _pp_mapping_pad;
> + * unsigned long dma_addr;
> + * atomic_long_t pp_ref_count;
> + * };
> + *
> + * We mirror the page_pool fields here so the page_pool can access these
> + * fields without worrying whether the underlying fields belong to a
> + * page or netmem_desc.
> + */
> +struct netmem_desc {
> + unsigned long _flags;
> + unsigned long pp_magic;
> + struct page_pool *pp;
> + unsigned long _pp_mapping_pad;
> + unsigned long dma_addr;
> + atomic_long_t pp_ref_count;
> +};
> +
> +#define NETMEM_DESC_ASSERT_OFFSET(pg, desc) \
> + static_assert(offsetof(struct page, pg) == \
> + offsetof(struct netmem_desc, desc))
> +NETMEM_DESC_ASSERT_OFFSET(flags, _flags);
> +NETMEM_DESC_ASSERT_OFFSET(pp_magic, pp_magic);
> +NETMEM_DESC_ASSERT_OFFSET(pp, pp);
> +NETMEM_DESC_ASSERT_OFFSET(_pp_mapping_pad, _pp_mapping_pad);
> +NETMEM_DESC_ASSERT_OFFSET(dma_addr, dma_addr);
> +NETMEM_DESC_ASSERT_OFFSET(pp_ref_count, pp_ref_count);
> +#undef NETMEM_DESC_ASSERT_OFFSET
> +
> +/*
> + * Since struct netmem_desc uses the space in struct page, the size
> + * should be checked, until struct netmem_desc has its own instance from
> + * slab, to avoid conflicting with other members within struct page.
> + */
> +static_assert(sizeof(struct netmem_desc) <= offsetof(struct page, _refcount));
> +
> /* net_iov */
>
> DECLARE_STATIC_KEY_FALSE(page_pool_mem_providers);
> @@ -31,12 +72,33 @@ enum net_iov_type {
> };
>
> struct net_iov {
> - enum net_iov_type type;
> - unsigned long pp_magic;
> - struct page_pool *pp;
> - struct net_iov_area *owner;
> - unsigned long dma_addr;
> - atomic_long_t pp_ref_count;
> + union {
> + struct netmem_desc desc;
> +
> + /* XXX: The following part should be removed once all
> + * the references to them are converted so as to be
> + * accessed via netmem_desc e.g. niov->desc.pp instead
> + * of niov->pp.
> + *
> + * Plus, once struct netmem_desc has it own instance
> + * from slab, network's fields of the following can be
> + * moved out of struct netmem_desc like:
> + *
> + * struct net_iov {
> + * struct netmem_desc desc;
> + * struct net_iov_area *owner;
> + * ...
> + * };
> + */

We do not need to wait until netmem_desc has its own instance from
slab to move the net_iov-specific fields out of netmem_desc. We can do
that now, because there are no size restrictions on net_iov.

So, I recommend change this to:

struct net_iov {
/* Union for anonymous aliasing: */
union {
struct netmem_desc desc;
struct {
unsigned long _flags;
unsigned long pp_magic;
struct page_pool *pp;
unsigned long _pp_mapping_pad;
unsigned long dma_addr;
atomic_long_t pp_ref_count;
};
struct net_iov_area *owner;
enum net_iov_type type;
};

>
> struct net_iov_area {
> @@ -48,27 +110,22 @@ struct net_iov_area {
> unsigned long base_virtual;
> };
>
> -/* These fields in struct page are used by the page_pool and net stack:
> +/* net_iov is union'ed with struct netmem_desc mirroring struct page, so
> + * the page_pool can access these fields without worrying whether the
> + * underlying fields are accessed via netmem_desc or directly via
> + * net_iov, until all the references to them are converted so as to be
> + * accessed via netmem_desc e.g. niov->desc.pp instead of niov->pp.
> *
> - * struct {
> - * unsigned long pp_magic;
> - * struct page_pool *pp;
> - * unsigned long _pp_mapping_pad;
> - * unsigned long dma_addr;
> - * atomic_long_t pp_ref_count;
> - * };
> - *
> - * We mirror the page_pool fields here so the page_pool can access these fields
> - * without worrying whether the underlying fields belong to a page or net_iov.
> - *
> - * The non-net stack fields of struct page are private to the mm stack and must
> - * never be mirrored to net_iov.
> + * The non-net stack fields of struct page are private to the mm stack
> + * and must never be mirrored to net_iov.
> */
> -#define NET_IOV_ASSERT_OFFSET(pg, iov) \
> - static_assert(offsetof(struct page, pg) == \
> +#define NET_IOV_ASSERT_OFFSET(desc, iov) \
> + static_assert(offsetof(struct netmem_desc, desc) == \
> offsetof(struct net_iov, iov))
> +NET_IOV_ASSERT_OFFSET(_flags, type);

Remove this assertion.

> NET_IOV_ASSERT_OFFSET(pp_magic, pp_magic);
> NET_IOV_ASSERT_OFFSET(pp, pp);
> +NET_IOV_ASSERT_OFFSET(_pp_mapping_pad, owner);

And this one.

(_flags, type) and (_pp_mapping_pad, owner) have very different
semantics and usage, we should not assert they are not the same
offset. However (pp, pp) and (pp_magic,pp_magic) have the same
semantics and usage, so we do assert they are at the same offset.

Code is allowed to access __netmem_clear_lsb(netmem)->pp or
__netmem_clear_lsb(netmem)->pp_magic without caring what's the
underlying memory type because both fields have the same semantics and
usage.

Code should *not* assume it can access
__netmem_clear_lsb(netmem)->owner or __netmem_clear_lsb(netmem)->type
without doing a check whether the underlying memory is
page/netmem_desc or net_iov. These fields are only usable for net_iov,
so let's explicitly move them to a different place.

--
Thanks,
Mina