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, 11 insertions, 0 deletions

diff --git a/net/core/xdp.c b/net/core/xdp.c
index f1165a35411b..a66a4e036f53 100644
--- a/net/core/xdp.c
+++ b/net/core/xdp.c
@@ -535,6 +535,17 @@ void xdp_return_frame_bulk(struct xdp_frame *xdpf,
 }
 EXPORT_SYMBOL_GPL(xdp_return_frame_bulk);
 
+/**
+ * xdp_return_frag -- free one XDP frag or decrement its refcount
+ * @netmem: network memory reference to release
+ * @xdp: &xdp_buff to release the frag for
+ */
+void xdp_return_frag(netmem_ref netmem, const struct xdp_buff *xdp)
+{
+	__xdp_return(netmem, xdp->rxq->mem.type, true, NULL);
+}
+EXPORT_SYMBOL_GPL(xdp_return_frag);
+
 void xdp_return_buff(struct xdp_buff *xdp)
 {
 	struct skb_shared_info *sinfo;