xdp: add generic xdp_buff_add_frag()

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 "oneliner".
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 x86_64) to track the "true" truesize. It can
be used later when updating the skb.

Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://patch.msgid.link/20241218174435.1445282-3-aleksander.lobakin@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 12 insertions, 4 deletions

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index b2509cd0b930..bb2b751d274a 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -608,11 +608,19 @@ 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];