[RFC v4 01/18] netmem: introduce struct netmem_desc mirroring struct page

Byungchul Park
2025-06-04T02:53:04Z

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.

Signed-off-by: Byungchul Park <byungchul@sk.com>
---
include/net/netmem.h | 94 ++++++++++++++++++++++++++++++++++----------
1 file changed, 73 insertions(+), 21 deletions(-)

diff --git a/include/net/netmem.h b/include/net/netmem.h
index 386164fb9c18..2687c8051ca5 100644
--- a/include/net/netmem.h
+++ b/include/net/netmem.h
@@ -12,6 +12,50 @@
#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.
+ *
+ * CAUTION: Do not update the fields in netmem_desc without also
+ * updating the anonymous aliasing union in struct net_iov.
+ */
+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 +75,25 @@ enum net_iov_type {
};

struct net_iov {
- enum net_iov_type type;
- unsigned long pp_magic;
- struct page_pool *pp;
+ 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.
+ */
+ 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;
- unsigned long dma_addr;
- atomic_long_t pp_ref_count;
+ enum net_iov_type type;
};

struct net_iov_area {
@@ -48,27 +105,22 @@ 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, _flags);
NET_IOV_ASSERT_OFFSET(pp_magic, pp_magic);
NET_IOV_ASSERT_OFFSET(pp, pp);
+NET_IOV_ASSERT_OFFSET(_pp_mapping_pad, _pp_mapping_pad);
NET_IOV_ASSERT_OFFSET(dma_addr, dma_addr);
NET_IOV_ASSERT_OFFSET(pp_ref_count, pp_ref_count);
#undef NET_IOV_ASSERT_OFFSET
--
2.17.1