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@intel.com>
---
 include/linux/skbuff.h | 16 ++++++--
 include/net/xdp.h      | 90 +++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 101 insertions(+), 5 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 0f0266183d3e..696b267ce15b 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -607,11 +607,19 @@ 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 {
+			u32		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 334814d377eb..ae92af8b782d 100644
--- a/include/net/xdp.h
+++ b/include/net/xdp.h
@@ -167,6 +167,88 @@ xdp_get_buff_len(const struct xdp_buff *xdp)
 	return len;
 }
 
+/**
+ * __xdp_buff_add_frag - attach a frag to an &xdp_buff
+ * @xdp: XDP buffer to attach the frag to
+ * @page: page containing the frag
+ * @offset: page offset at which the frag starts
+ * @size: size of the frag
+ * @truesize: truesize (page / page frag size) of the frag
+ * @try_coalesce: whether to try coalescing the frags
+ *
+ * Attach a frag to an XDP buffer. If it currently has no frags attached,
+ * initialize the related fields, otherwise check that the frag number
+ * didn't reach the limit of ``MAX_SKB_FRAGS``. If possible, try coalescing
+ * the frag with the previous one.
+ * The function doesn't check/update the pfmemalloc bit. Please use the
+ * non-underscored wrapper in drivers.
+ *
+ * Return: true on success, false if there's no space for the frag in
+ * the shared info struct.
+ */
+static inline bool __xdp_buff_add_frag(struct xdp_buff *xdp, struct page *page,
+				       u32 offset, u32 size, u32 truesize,
+				       bool try_coalesce)
+{
+	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
+	skb_frag_t *prev;
+	u32 nr_frags;
+
+	if (!xdp_buff_has_frags(xdp)) {
+		xdp_buff_set_frags_flag(xdp);
+
+		nr_frags = 0;
+		sinfo->xdp_frags_size = 0;
+		sinfo->xdp_frags_truesize = 0;
+
+		goto fill;
+	}
+
+	nr_frags = sinfo->nr_frags;
+	if (unlikely(nr_frags == MAX_SKB_FRAGS))
+		return false;
+
+	prev = &sinfo->frags[nr_frags - 1];
+	if (try_coalesce && page == skb_frag_page(prev) &&
+	    offset == skb_frag_off(prev) + skb_frag_size(prev))
+		skb_frag_size_add(prev, size);
+	else
+fill:
+		__skb_fill_page_desc_noacc(sinfo, nr_frags++, page,
+					   offset, size);
+
+	sinfo->nr_frags = nr_frags;
+	sinfo->xdp_frags_size += size;
+	sinfo->xdp_frags_truesize += truesize;
+
+	return true;
+}
+
+/**
+ * xdp_buff_add_frag - attach a frag to an &xdp_buff
+ * @xdp: XDP buffer to attach the frag to
+ * @page: page containing the frag
+ * @offset: page offset at which the frag starts
+ * @size: size of the frag
+ * @truesize: truesize (page / page frag size) of the frag
+ *
+ * Version of __xdp_buff_add_frag() which takes care of the pfmemalloc bit.
+ *
+ * Return: true on success, false if there's no space for the frag in
+ * the shared info struct.
+ */
+static inline bool xdp_buff_add_frag(struct xdp_buff *xdp, struct page *page,
+				     u32 offset, u32 size, u32 truesize)
+{
+	if (!__xdp_buff_add_frag(xdp, page, offset, size, truesize, true))
+		return false;
+
+	if (unlikely(page_is_pfmemalloc(page)))
+		xdp_buff_set_frag_pfmemalloc(xdp);
+
+	return true;
+}
+
 struct xdp_frame {
 	void *data;
 	u32 len;
@@ -230,7 +312,13 @@ 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;
-- 
2.46.2