xdp: Cpumap redirect use frame_sz and increase skb_tailroom (db612f74) · Commits · 戴 / test

Knowing the memory size backing the packet/xdp_frame data area, and knowing it already have reserved room for skb_shared_info, simplifies using build_skb significantly. With this change we no-longer lie about the SKB truesize, but more importantly a significant larger skb_tailroom is now provided, e.g. when drivers uses a full PAGE_SIZE. This extra tailroom (in linear area) can be used by the network stack when coalescing SKBs (e.g. in skb_try_coalesce, see TCP cases where tcp_queue_rcv() can 'eat' skb). Signed-off-by:

Jesper Dangaard Brouer <brouer@redhat.com> Signed-off-by:

Alexei Starovoitov <ast@kernel.org> Acked-by:

Toke Høiland-Jørgensen <toke@redhat.com> Link: https://lore.kernel.org/bpf/158945337822.97035.13557959180460986059.stgit@firesoul

kernel/bpf/cpumap.c

+3 −18

Original line number	Diff line number	Diff line
		@@ -162,25 +162,10 @@ static struct sk_buff cpu_map_build_skb(struct bpf_cpu_map_entry rcpu,
		/* Part of headroom was reserved to xdpf */
		hard_start_headroom = sizeof(struct xdp_frame) + xdpf->headroom;

		/* build_skb need to place skb_shared_info after SKB end, and
		* also want to know the memory "truesize". Thus, need to
		* know the memory frame size backing xdp_buff.
		*
		* XDP was designed to have PAGE_SIZE frames, but this
		* assumption is not longer true with ixgbe and i40e. It
		* would be preferred to set frame_size to 2048 or 4096
		* depending on the driver.
		* frame_size = 2048;
		* frame_len = frame_size - sizeof(*xdp_frame);
		*
		* Instead, with info avail, skb_shared_info in placed after
		* packet len. This, unfortunately fakes the truesize.
		* Another disadvantage of this approach, the skb_shared_info
		* is not at a fixed memory location, with mixed length
		* packets, which is bad for cache-line hotness.
		/* Memory size backing xdp_frame data already have reserved
		* room for build_skb to place skb_shared_info in tailroom.
		*/
		frame_size = SKB_DATA_ALIGN(xdpf->len + hard_start_headroom) +
		SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
		frame_size = xdpf->frame_sz;

		pkt_data_start = xdpf->data - hard_start_headroom;
		skb = build_skb_around(skb, pkt_data_start, frame_size);

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