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

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Date: Mon, 17 Jun 2024 14:10:37 +0200
From: Ilya Maximets <i.maximets@....org>
To: Adrian Moreno <amorenoz@...hat.com>, netdev@...r.kernel.org
Cc: i.maximets@....org, aconole@...hat.com, echaudro@...hat.com,
 horms@...nel.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 6/17/24 13:55, Ilya Maximets wrote:
> On 6/3/24 20:56, 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>
>> ---
>>  net/openvswitch/actions.c | 13 +++++++++++--
>>  1 file changed, 11 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c
>> index 33f6d93ba5e4..54fc1abcff95 100644
>> --- a/net/openvswitch/actions.c
>> +++ b/net/openvswitch/actions.c
>> @@ -82,6 +82,15 @@ static struct action_fifo __percpu *action_fifos;
>>  static struct action_flow_keys __percpu *flow_keys;
>>  static DEFINE_PER_CPU(int, exec_actions_level);
>>  
>> +static inline void ovs_drop_skb_last_action(struct sk_buff *skb)
>> +{
>> +	/* Do not emit packet drops inside sample(). */
>> +	if (OVS_CB(skb)->probability)
>> +		consume_skb(skb);
>> +	else
>> +		ovs_kfree_skb_reason(skb, OVS_DROP_LAST_ACTION);
>> +}
>> +
>>  /* Make a clone of the 'key', using the pre-allocated percpu 'flow_keys'
>>   * space. Return NULL if out of key spaces.
>>   */
>> @@ -1061,7 +1070,7 @@ static int sample(struct datapath *dp, struct sk_buff *skb,
>>  	if ((arg->probability != U32_MAX) &&
>>  	    (!arg->probability || get_random_u32() > arg->probability)) {
>>  		if (last)
>> -			ovs_kfree_skb_reason(skb, OVS_DROP_LAST_ACTION);
>> +			ovs_drop_skb_last_action(skb);

Always consuming the skb at this point makes sense, since having smaple()
as a last action is a reasonable thing to have.  But this looks more like
a fix for the original drop reason patch set.

>>  		return 0;
>>  	}
>>  
>> @@ -1579,7 +1588,7 @@ static int do_execute_actions(struct datapath *dp, struct sk_buff *skb,
>>  		}
>>  	}
>>  
>> -	ovs_kfree_skb_reason(skb, OVS_DROP_LAST_ACTION);
>> +	ovs_drop_skb_last_action(skb);
> 
> I don't think I agree with this one.  If we have a sample() action with
> a lot of different actions inside and we reached the end while the last
> action didn't consume the skb, then we should report that.  E.g.
> "sample(emit_sample(),push_vlan(),set(eth())),2"  should report that the
> cloned skb was dropped.  "sample(push_vlan(),emit_sample())" should not.
> 
> The only actions that are actually consuming the skb are "output",
> "userspace", "recirc" and now "emit_sample".  "output" and "recirc" are
> consuming the skb "naturally" by stealing it when it is the last action.
> "userspace" has an explicit check to consume the skb if it is the last
> action.  "emit_sample" should have the similar check.  It should likely
> be added at the point of action introduction instead of having a separate
> patch.
> 
> Best regards, Ilya Maximets.