netdev - [PATCH net-next 05/10] udp: refine __udp_enqueue_schedule

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Message-ID: <20250916160951.541279-6-edumazet@google.com>
Date: Tue, 16 Sep 2025 16:09:46 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: Simon Horman <horms@...nel.org>, Willem de Bruijn <willemb@...gle.com>, 
	Kuniyuki Iwashima <kuniyu@...gle.com>, David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com, Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net-next 05/10] udp: refine __udp_enqueue_schedule_skb() test

Commit 5a465a0da13e ("udp: Fix multiple wraparounds
of sk->sk_rmem_alloc.") allowed to slightly overshoot
sk->sk_rmem_alloc, when many cpus are trying
to feed packets to a common UDP socket.

This patch, combined with the following one reduces
false sharing on the victim socket under DDOS.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 net/ipv4/udp.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index cca41c569f37621404829e096306ba7d78ce4d43..edd846fee90ff7850356a5cb3400ce96856e5429 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1739,8 +1739,8 @@ int __udp_enqueue_schedule_skb(struct sock *sk, struct sk_buff *skb)
 		if (rcvbuf > INT_MAX >> 1)
 			goto drop;
 
-		/* Always allow at least one packet for small buffer. */
-		if (rmem > rcvbuf)
+		/* Accept the packet if queue is empty. */
+		if (rmem)
 			goto drop;
 	}
 
-- 
2.51.0.384.g4c02a37b29-goog