[PATCH RFC net-next 12/34] xdp: add generic xdp_buff_add_frag()

[Alexander Lobakin]

The code piece which would attach a frag to &xdp_buff is almost
identical across the drivers supporting XDP multi-buffer on Rx.
Make it a generic elegant onelner.
Also, I see lots of drivers calculating frags_truesize as
`xdp->frame_sz * nr_frags`. I can't say this is fully correct, since
frags might be backed by chunks of different sizes, especially with
stuff like the header split. Even page_pool_alloc() can give you two
different truesizes on two subsequent requests to allocate the same
buffer size. Add a field to &skb_shared_info (unionized as there's no
free slot currently on x6_64) to track the "true" truesize. It can be
used later when updating an skb.

Signed-off-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
---
 include/linux/skbuff.h | 14 ++++++++++----
 include/net/xdp.h      | 36 +++++++++++++++++++++++++++++++++++-
 2 files changed, 45 insertions(+), 5 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index ea5c8ab3ed00..e350efa04070 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -598,11 +598,17 @@ struct skb_shared_info {
 	 * Warning : all fields before dataref are cleared in __alloc_skb()
 	 */
 	atomic_t	dataref;
-	unsigned int	xdp_frags_size;
 
-	/* Intermediate layers must ensure that destructor_arg
-	 * remains valid until skb destructor */
-	void *		destructor_arg;
+	union {
+		struct {
+			unsigned int	xdp_frags_size;
+			u32		xdp_frags_truesize;
+		};
+
+		/* Intermediate layers must ensure that destructor_arg
+		 * remains valid until skb destructor */
+		void *		destructor_arg;
+	};
 
 	/* must be last field, see pskb_expand_head() */
 	skb_frag_t	frags[MAX_SKB_FRAGS];
diff --git a/include/net/xdp.h b/include/net/xdp.h
index 909c0bc50517..a3dc0f39b437 100644
--- a/include/net/xdp.h
+++ b/include/net/xdp.h
@@ -165,6 +165,34 @@ xdp_get_buff_len(const struct xdp_buff *xdp)
 	return len;
 }
 
+static inline bool xdp_buff_add_frag(struct xdp_buff *xdp, struct page *page,
+				     u32 offset, u32 size, u32 truesize)
+{
+	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
+
+	if (!xdp_buff_has_frags(xdp)) {
+		sinfo->nr_frags = 0;
+
+		sinfo->xdp_frags_size = 0;
+		sinfo->xdp_frags_truesize = 0;
+
+		xdp_buff_set_frags_flag(xdp);
+	}
+
+	if (unlikely(sinfo->nr_frags == MAX_SKB_FRAGS))
+		return false;
+
+	__skb_fill_page_desc_noacc(sinfo, sinfo->nr_frags++, page, offset,
+				   size);
+	sinfo->xdp_frags_size += size;
+	sinfo->xdp_frags_truesize += truesize;
+
+	if (unlikely(page_is_pfmemalloc(page)))
+		xdp_buff_set_frag_pfmemalloc(xdp);
+
+	return true;
+}
+
 struct xdp_frame {
 	void *data;
 	u16 len;
@@ -230,7 +258,13 @@ xdp_update_skb_shared_info(struct sk_buff *skb, u8 nr_frags,
 			   unsigned int size, unsigned int truesize,
 			   bool pfmemalloc)
 {
-	skb_shinfo(skb)->nr_frags = nr_frags;
+	struct skb_shared_info *sinfo = skb_shinfo(skb);
+
+	sinfo->nr_frags = nr_frags;
+	/* ``destructor_arg`` is unionized with ``xdp_frags_{,true}size``,
+	 * reset it after that these fields aren't used anymore.
+	 */
+	sinfo->destructor_arg = NULL;
 
 	skb->len += size;
 	skb->data_len += size;
-- 
2.43.0