[PATCH net-next v2 3/4] net: add skb_frags_can_coalesce() helper

[Mina Almasry]

Add skb_has_net_iovs(skb), assuming skb->unreadable and net_iov-backed
frags are equivalent for now (with a comment noting the two can be
decoupled in the future if readable net_iovs are introduced).

Also add skb_mem_provider_compat() and skb_frags_can_coalesce() so all
multi-skb coalescing, shifting, and GRO paths can share a single check
across pp_recycle, skb_zcopy(), and fragment memory providers:
- empty 0-length skbs are compatible without mixing readable payload
- page vs net_iov is rejected via skb_has_net_iovs() (0 pointer chases)
- page vs page is allowed via !skb_has_net_iovs() (0 pointer chases)
- net_iov vs net_iov compares mp_priv (walking frag_list if nr_frags is 0)

Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx>
---
 include/linux/skbuff.h | 58 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 58 insertions(+)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 6787fcf6ac3e4..1e6e919c656d5 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1882,6 +1882,16 @@ static inline bool skb_frags_readable(const struct sk_buff *skb)
 	return !skb->unreadable;
 }
 
+/* Return true if frags in this skb are backed by net_iovs.
+ * Assumes skb->unreadable and net_iov-backed frags are equivalent for now;
+ * the two can be decoupled in the future if readable net_iovs in skbs are
+ * introduced.
+ */
+static inline bool skb_has_net_iovs(const struct sk_buff *skb)
+{
+	return skb->unreadable;
+}
+
 static inline void skb_mark_not_on_list(struct sk_buff *skb)
 {
 	skb->next = NULL;
@@ -3763,6 +3773,54 @@ static inline void *skb_frag_address(const skb_frag_t *frag)
 	return page_address(skb_frag_page(frag)) + skb_frag_off(frag);
 }
 
+static inline void *skb_mem_provider_priv(const struct sk_buff *skb)
+{
+	netmem_ref netmem;
+
+	if (!skb_has_net_iovs(skb) || !skb_shinfo(skb)->nr_frags)
+		return NULL;
+
+	netmem = skb_frag_netmem(&skb_shinfo(skb)->frags[0]);
+	return netmem_to_net_iov(netmem)->mp_priv;
+}
+
+/* Return true if the memory providers of @skb1 and @skb2 are compatible
+ * for coalescing. For any struct page-backed provider (the buddy allocator
+ * or a future provider handing out struct pages), coalescing is assumed
+ * feasible. For net_iov-backed providers, the exact mp_priv pointer must
+ * match.
+ */
+static inline bool skb_mem_provider_compat(const struct sk_buff *skb1,
+					   const struct sk_buff *skb2)
+{
+	void *priv1, *priv2;
+
+	if (!skb1->len || !skb2->len)
+		return true;
+
+	if (skb_has_net_iovs(skb1) != skb_has_net_iovs(skb2))
+		return false;
+
+	if (!skb_has_net_iovs(skb1))
+		return true;
+
+	priv1 = skb_mem_provider_priv(skb1);
+	priv2 = skb_mem_provider_priv(skb2);
+	return priv1 && priv1 == priv2;
+}
+
+static inline bool skb_frags_can_coalesce(struct sk_buff *to,
+					  struct sk_buff *from)
+{
+	if (to->pp_recycle != from->pp_recycle)
+		return false;
+
+	if (skb_zcopy(to) || skb_zcopy(from))
+		return false;
+
+	return skb_mem_provider_compat(to, from);
+}
+
 /**
  * skb_frag_address_safe - gets the address of the data contained in a paged fragment
  * @frag: the paged fragment buffer
-- 
2.56.0.385.gac671c819a-goog