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

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Message-ID: <aSgK2wfLJurn2df5@shredder>
Date: Thu, 27 Nov 2025 10:24:59 +0200
From: Ido Schimmel <idosch@...dia.com>
To: Jon Kohler <jon@...anix.com>
Cc: "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>,
	Jason Wang <jasowang@...hat.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] flow_dissector: save computed hash in
 __skb_get_hash_symmetric_net()

On Wed, Nov 26, 2025 at 04:21:33PM +0000, Jon Kohler wrote:
> What about a variant of this patch that had an arg like:
> __skb_get_hash_symmetric(struct sk_buff *skb, bool save_hash)
> 
> Then we just make calls (like tun) opt in?

It will require changes in all the callers and I am not sure it's wise
to change a common function for a single user. Why not just patch tun to
call __skb_set_sw_hash(skb, hash, true)? IIUC, even in tun you only need
it in two out of the four callers of __skb_get_hash_symmetric():
tun_get_user() and tun_xdp_one() which both build an skb before
injecting it into the Rx path.