netdev - [PATCH bpf-next 01/13] bpf: in bpf_skb_adjust

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Message-Id: <20190320144944.147862-2-willemdebruijn.kernel@gmail.com>
Date:   Wed, 20 Mar 2019 10:49:32 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     netdev@...r.kernel.org
Cc:     ast@...nel.org, daniel@...earbox.net, sdf@...gle.com,
        posk@...gle.com, Willem de Bruijn <willemb@...gle.com>
Subject: [PATCH bpf-next 01/13] bpf: in bpf_skb_adjust_room avoid copy in tx fast path

From: Willem de Bruijn <willemb@...gle.com>

bpf_skb_adjust_room calls skb_cow on grow.

This expensive operation can be avoided in the fast path when the only
other clone has released the header. This is the common case for TCP,
where one headerless clone is kept on the retransmit queue.

It is safe to do so even when touching the gso fields in skb_shinfo.
Regular tunnel encap with iptunnel_handle_offloads takes the same
optimization.

The tcp stack unclones in the unlikely case that it accesses these
fields through headerless clones packets on the retransmit queue (see
__tcp_retransmit_skb).

If any other clones are present, e.g., from packet sockets,
skb_cow_head returns the same value as skb_cow().

Signed-off-by: Willem de Bruijn <willemb@...gle.com>
---
 net/core/filter.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 647c63a7b25b6..8e15fb919b574 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2971,7 +2971,7 @@ static int bpf_skb_net_grow(struct sk_buff *skb, u32 len_diff)
 	if (skb_is_gso(skb) && !skb_is_gso_tcp(skb))
 		return -ENOTSUPP;

-	ret = skb_cow(skb, len_diff);
+	ret = skb_cow_head(skb, len_diff);
 	if (unlikely(ret < 0))
 		return ret;

-- 
2.21.0.225.g810b269d1ac-goog