net: add get_netmem/put_netmem support

Currently net_iovs support only pp ref counts, and do not support a
page ref equivalent.

This is fine for the RX path as net_iovs are used exclusively with the
pp and only pp refcounting is needed there. The TX path however does not
use pp ref counts, thus, support for get_page/put_page equivalent is
needed for netmem.

Support get_netmem/put_netmem. Check the type of the netmem before
passing it to page or net_iov specific code to obtain a page ref
equivalent.

For dmabuf net_iovs, we obtain a ref on the underlying binding. This
ensures the entire binding doesn't disappear until all the net_iovs have
been put_netmem'ed. We do not need to track the refcount of individual
dmabuf net_iovs as we don't allocate/free them from a pool similar to
what the buddy allocator does for pages.

This code is written to be extensible by other net_iov implementers.
get_netmem/put_netmem will check the type of the netmem and route it to
the correct helper:

pages -> [get|put]_page()
dmabuf net_iovs -> net_devmem_[get|put]_net_iov()
new net_iovs ->	new helpers

Signed-off-by: Mina Almasry <almasrymina@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250508004830.4100853-3-almasrymina@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/linux/skbuff_ref.h

@@ -17,7 +17,7 @@
  */
 static inline void __skb_frag_ref(skb_frag_t *frag)
 {
-	get_page(skb_frag_page(frag));
+	get_netmem(skb_frag_netmem(frag));
 }
 
 /**
@@ -40,7 +40,7 @@ static inline void skb_page_unref(netmem_ref netmem, bool recycle)
 	if (recycle && napi_pp_put_page(netmem))
 		return;
 #endif
-	put_page(netmem_to_page(netmem));
+	put_netmem(netmem);
 }
 
 /**

include/net/netmem.h

@@ -273,4 +273,7 @@ static inline unsigned long netmem_get_dma_addr(netmem_ref netmem)
 	return __netmem_clear_lsb(netmem)->dma_addr;
 }
 
+void get_netmem(netmem_ref netmem);
+void put_netmem(netmem_ref netmem);
+
 #endif /* _NET_NETMEM_H */

net/core/devmem.c

@@ -295,6 +295,16 @@ err_put_dmabuf:
 	return ERR_PTR(err);
 }
 
+void net_devmem_get_net_iov(struct net_iov *niov)
+{
+	net_devmem_dmabuf_binding_get(net_devmem_iov_binding(niov));
+}
+
+void net_devmem_put_net_iov(struct net_iov *niov)
+{
+	net_devmem_dmabuf_binding_put(net_devmem_iov_binding(niov));
+}
+
 /*** "Dmabuf devmem memory provider" ***/
 
 int mp_dmabuf_devmem_init(struct page_pool *pool)

net/core/devmem.h

@@ -29,6 +29,10 @@ struct net_devmem_dmabuf_binding {
 	 * The binding undos itself and unmaps the underlying dmabuf once all
 	 * those refs are dropped and the binding is no longer desired or in
 	 * use.
+	 *
+	 * net_devmem_get_net_iov() on dmabuf net_iovs will increment this
+	 * reference, making sure that the binding remains alive until all the
+	 * net_iovs are no longer used.
 	 */
 	refcount_t ref;
 
@@ -111,6 +115,9 @@ net_devmem_dmabuf_binding_put(struct net_devmem_dmabuf_binding *binding)
 	__net_devmem_dmabuf_binding_free(binding);
 }
 
+void net_devmem_get_net_iov(struct net_iov *niov);
+void net_devmem_put_net_iov(struct net_iov *niov);
+
 struct net_iov *
 net_devmem_alloc_dmabuf(struct net_devmem_dmabuf_binding *binding);
 void net_devmem_free_dmabuf(struct net_iov *ppiov);
@@ -120,6 +127,19 @@ bool net_is_devmem_iov(struct net_iov *niov);
 #else
 struct net_devmem_dmabuf_binding;
 
+static inline void
+net_devmem_dmabuf_binding_put(struct net_devmem_dmabuf_binding *binding)
+{
+}
+
+static inline void net_devmem_get_net_iov(struct net_iov *niov)
+{
+}
+
+static inline void net_devmem_put_net_iov(struct net_iov *niov)
+{
+}
+
 static inline void
 __net_devmem_dmabuf_binding_free(struct net_devmem_dmabuf_binding *binding)
 {

net/core/skbuff.c

@@ -89,6 +89,7 @@
 #include <linux/textsearch.h>
 
 #include "dev.h"
+#include "devmem.h"
 #include "netmem_priv.h"
 #include "sock_destructor.h"
 
@@ -7313,3 +7314,32 @@ bool csum_and_copy_from_iter_full(void *addr, size_t bytes,
 	return false;
 }
 EXPORT_SYMBOL(csum_and_copy_from_iter_full);
+
+void get_netmem(netmem_ref netmem)
+{
+	struct net_iov *niov;
+
+	if (netmem_is_net_iov(netmem)) {
+		niov = netmem_to_net_iov(netmem);
+		if (net_is_devmem_iov(niov))
+			net_devmem_get_net_iov(netmem_to_net_iov(netmem));
+		return;
+	}
+	get_page(netmem_to_page(netmem));
+}
+EXPORT_SYMBOL(get_netmem);
+
+void put_netmem(netmem_ref netmem)
+{
+	struct net_iov *niov;
+
+	if (netmem_is_net_iov(netmem)) {
+		niov = netmem_to_net_iov(netmem);
+		if (net_is_devmem_iov(niov))
+			net_devmem_put_net_iov(netmem_to_net_iov(netmem));
+		return;
+	}
+
+	put_page(netmem_to_page(netmem));
+}
+EXPORT_SYMBOL(put_netmem);