netdev - Re: [PATCH net-next v2 7/9] net: openvswitch: do not notify drops inside sample

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Date: Fri, 14 Jun 2024 17:17:08 +0100
From: Simon Horman <horms@...nel.org>
To: Adrian Moreno <amorenoz@...hat.com>
Cc: netdev@...r.kernel.org, aconole@...hat.com, echaudro@...hat.com,
	i.maximets@....org, dev@...nvswitch.org,
	Pravin B Shelar <pshelar@....org>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v2 7/9] net: openvswitch: do not notify drops
 inside sample

On Mon, Jun 03, 2024 at 08:56:41PM +0200, Adrian Moreno wrote:
> The OVS_ACTION_ATTR_SAMPLE action is, in essence,
> observability-oriented.
> 
> Apart from some corner case in which it's used a replacement of clone()
> for old kernels, it's really only used for sFlow, IPFIX and now,
> local emit_sample.
> 
> With this in mind, it doesn't make much sense to report
> OVS_DROP_LAST_ACTION inside sample actions.
> 
> For instance, if the flow:
> 
>   actions:sample(..,emit_sample(..)),2
> 
> triggers a OVS_DROP_LAST_ACTION skb drop event, it would be extremely
> confusing for users since the packet did reach its destination.
> 
> This patch makes internal action execution silently consume the skb
> instead of notifying a drop for this case.
> 
> Unfortunately, this patch does not remove all potential sources of
> confusion since, if the sample action itself is the last action, e.g:
> 
>     actions:sample(..,emit_sample(..))
> 
> we actually _should_ generate a OVS_DROP_LAST_ACTION event, but we aren't.
> 
> Sadly, this case is difficult to solve without breaking the
> optimization by which the skb is not cloned on last sample actions.
> But, given explicit drop actions are now supported, OVS can just add one
> after the last sample() and rewrite the flow as:
> 
>     actions:sample(..,emit_sample(..)),drop
> 
> Signed-off-by: Adrian Moreno <amorenoz@...hat.com>

Reviewed-by: Simon Horman <horms@...nel.org>