[RFC PATCH net-next v1 3/4] net: add netmem_t to skb_frag_t

[Mina Almasry]

Use netmem_t instead of page directly in skb_frag_t. Currently netmem_t
is always a struct page underneath, but the abstraction allows efforts
to add support for skb frags not backed by pages.

There is unfortunately 1 instance where the skb_frag_t is assumed to be
a bio_vec in kcm. For this case, add a debug assert that the skb frag is
indeed backed by a page, and do a cast.

Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx>
---
 include/linux/skbuff.h | 11 ++++++++---
 net/kcm/kcmsock.c      |  9 +++++++--
 2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index b370eb8d70f7..6d681c40213c 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -37,6 +37,7 @@
 #endif
 #include <net/net_debug.h>
 #include <net/dropreason-core.h>
+#include <net/netmem.h>
 
 /**
  * DOC: skb checksums
@@ -359,7 +360,11 @@ extern int sysctl_max_skb_frags;
  */
 #define GSO_BY_FRAGS	0xFFFF
 
-typedef struct bio_vec skb_frag_t;
+typedef struct skb_frag {
+	struct netmem *bv_page;
+	unsigned int bv_len;
+	unsigned int bv_offset;
+} skb_frag_t;
 
 /**
  * skb_frag_size() - Returns the size of a skb fragment
@@ -2435,7 +2440,7 @@ static inline void skb_frag_fill_page_desc(skb_frag_t *frag,
 					   struct page *page,
 					   int off, int size)
 {
-	frag->bv_page = page;
+	frag->bv_page = page_to_netmem(page);
 	frag->bv_offset = off;
 	skb_frag_size_set(frag, size);
 }
@@ -3422,7 +3427,7 @@ static inline void skb_frag_off_copy(skb_frag_t *fragto,
  */
 static inline struct page *skb_frag_page(const skb_frag_t *frag)
 {
-	return frag->bv_page;
+	return netmem_to_page(frag->bv_page);
 }
 
 /**
diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
index 65d1f6755f98..926349eeeaf6 100644
--- a/net/kcm/kcmsock.c
+++ b/net/kcm/kcmsock.c
@@ -636,9 +636,14 @@ static int kcm_write_msgs(struct kcm_sock *kcm)
 		for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
 			msize += skb_shinfo(skb)->frags[i].bv_len;
 
+		/* The cast to struct bio_vec* here assumes the frags are
+		 * struct page based.
+		 */
+		DEBUG_NET_WARN_ON_ONCE(!skb_frag_page(&skb_shinfo(skb)->frags[0]));
+
 		iov_iter_bvec(&msg.msg_iter, ITER_SOURCE,
-			      skb_shinfo(skb)->frags, skb_shinfo(skb)->nr_frags,
-			      msize);
+			      (const struct bio_vec *)skb_shinfo(skb)->frags,
+			      skb_shinfo(skb)->nr_frags, msize);
 		iov_iter_advance(&msg.msg_iter, txm->frag_offset);
 
 		do {
-- 
2.43.0.472.g3155946c3a-goog