netdev - [PATCH net] net: tcp: don't allocate fast clones for fastopen SYN

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Message-Id: <20210302060753.953931-1-kuba@kernel.org>
Date:   Mon,  1 Mar 2021 22:07:53 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     davem@...emloft.net
Cc:     netdev@...r.kernel.org, edumazet@...gle.com, kernel-team@...com,
        Jakub Kicinski <kuba@...nel.org>, Neil Spring <ntspring@...com>
Subject: [PATCH net] net: tcp: don't allocate fast clones for fastopen SYN

When receiver does not accept TCP Fast Open it will only ack
the SYN, and not the data. We detect this and immediately queue
the data for (re)transmission in tcp_rcv_fastopen_synack().

In DC networks with very low RTT and without RFS the SYN-ACK
may arrive before NIC driver reported Tx completion on
the original SYN. In which case skb_still_in_host_queue()
returns true and sender will need to wait for the retransmission
timer to fire milliseconds later.

Revert back to non-fast clone skbs, this way
skb_still_in_host_queue() won't prevent the recovery flow
from completing.

Suggested-by: Eric Dumazet <edumazet@...gle.com>
Fixes: 355a901e6cf1 ("tcp: make connect() mem charging friendly")
Signed-off-by: Neil Spring <ntspring@...com>
Signed-off-by: Jakub Kicinski <kuba@...nel.org>
---
 net/ipv4/tcp_output.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index fbf140a770d8..cd9461588539 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3759,9 +3759,16 @@ static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn)
 	/* limit to order-0 allocations */
 	space = min_t(size_t, space, SKB_MAX_HEAD(MAX_TCP_HEADER));
 
-	syn_data = sk_stream_alloc_skb(sk, space, sk->sk_allocation, false);
+	syn_data = alloc_skb(MAX_TCP_HEADER + space, sk->sk_allocation);
 	if (!syn_data)
 		goto fallback;
+	if (!sk_wmem_schedule(sk, syn_data->truesize)) {
+		__kfree_skb(syn_data);
+		goto fallback;
+	}
+	skb_reserve(syn_data, MAX_TCP_HEADER);
+	INIT_LIST_HEAD(&syn_data->tcp_tsorted_anchor);
+
 	syn_data->ip_summed = CHECKSUM_PARTIAL;
 	memcpy(syn_data->cb, syn->cb, sizeof(syn->cb));
 	if (space) {
-- 
2.26.2