linux-kernel - [PATCH net-next] flow_dissector: save computed hash in __skb_get_hash_symmetric

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Message-ID: <20251125181930.1192165-1-jon@nutanix.com>
Date: Tue, 25 Nov 2025 11:19:27 -0700
From: Jon Kohler <jon@...anix.com>
To: "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        Jon Kohler <jon@...anix.com>, Ido Schimmel <idosch@...dia.com>,
        Jason Wang <jasowang@...hat.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH net-next] flow_dissector: save computed hash in __skb_get_hash_symmetric_net()

tun.c changed from skb_get_hash() to __skb_get_hash_symmetric() on
commit feec084a7cf4 ("tun: use symmetric hash"), which exposes an
overhead for OVS datapath, where ovs_dp_process_packet() has to
calculate the hash again because __skb_get_hash_symmetric() does not
retain the hash that it calculates.

Save the computed hash in __skb_get_hash_symmetric_net() so that the
calcuation work does not go to waste.

Fixes: feec084a7cf4 ("tun: use symmetric hash")
Signed-off-by: Jon Kohler <jon@...anix.com>
---
 include/linux/skbuff.h    | 4 ++--
 net/core/flow_dissector.c | 7 +++++--
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index ff90281ddf90..f58afa49a50e 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1568,9 +1568,9 @@ __skb_set_sw_hash(struct sk_buff *skb, __u32 hash, bool is_l4)
 	__skb_set_hash(skb, hash, true, is_l4);
 }
 
-u32 __skb_get_hash_symmetric_net(const struct net *net, const struct sk_buff *skb);
+u32 __skb_get_hash_symmetric_net(const struct net *net, struct sk_buff *skb);
 
-static inline u32 __skb_get_hash_symmetric(const struct sk_buff *skb)
+static inline u32 __skb_get_hash_symmetric(struct sk_buff *skb)
 {
 	return __skb_get_hash_symmetric_net(NULL, skb);
 }
diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 1b61bb25ba0e..4a74dcc4799c 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -1864,9 +1864,10 @@ EXPORT_SYMBOL(make_flow_keys_digest);
 
 static struct flow_dissector flow_keys_dissector_symmetric __read_mostly;
 
-u32 __skb_get_hash_symmetric_net(const struct net *net, const struct sk_buff *skb)
+u32 __skb_get_hash_symmetric_net(const struct net *net, struct sk_buff *skb)
 {
 	struct flow_keys keys;
+	u32 flow_hash;
 
 	__flow_hash_secret_init();
 
@@ -1874,7 +1875,9 @@ u32 __skb_get_hash_symmetric_net(const struct net *net, const struct sk_buff *sk
 	__skb_flow_dissect(net, skb, &flow_keys_dissector_symmetric,
 			   &keys, NULL, 0, 0, 0, 0);
 
-	return __flow_hash_from_keys(&keys, &hashrnd);
+	flow_hash = __flow_hash_from_keys(&keys, &hashrnd);
+	__skb_set_sw_hash(skb, flow_hash, flow_keys_have_l4(&keys));
+	return flow_hash;
 }
 EXPORT_SYMBOL_GPL(__skb_get_hash_symmetric_net);
 
-- 
2.43.0