netdev - Re: [PATCH net-next 1/2 v2] net sched actions: dump more than TCA_ACT_MAX

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Date:   Tue, 18 Apr 2017 20:17:25 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jamal Hadi Salim <jhs@...atatu.com>
Cc:     davem@...emloft.net, netdev@...r.kernel.org, jiri@...nulli.us,
        xiyou.wangcong@...il.com
Subject: Re: [PATCH net-next 1/2 v2] net sched actions: dump more than
 TCA_ACT_MAX_PRIO actions per batch

On Tue, 2017-04-18 at 22:32 -0400, Jamal Hadi Salim wrote:
> On 17-04-18 09:49 PM, Eric Dumazet wrote:
> > On Tue, 2017-04-18 at 21:14 -0400, Jamal Hadi Salim wrote:
> >> From: Jamal Hadi Salim <jhs@...atatu.com>
> >
> > So there is no more limit ? How user is supposed to size the buffer for
> > recvmsg() ?
> >
> 
> That part doesnt change.  Ok, I believe more clarity is needed:->
> 
> Current code allows only 32 actions to be dumped at a time.
> This code:
> 
> So lets i have 128 actions. I can fit them into 10K.
> I do recvmsg(..., 32K). This will make about 32K space for me to fill
> up the 128 actions. But the code only allows me to send 32
> TCA_ACT_MAX_PRIO which takes about 2-3K.
> So I will do 4 runs from kernel->user to get 128 actions ;->
> User space in tc is trained to expect no more than
> TCA_ACT_MAX_PRIO in every batch.
> 
> With this change I ask the kernel to fit as many actions as
> possible in the 32K (all these 128 will fit in one batch).
> Then it has to tell user space how many are in that batch
> using TCAA_ACT_COUNT attribute.
> 
> Make sense?

What if we have 1024 actions, and user provides a 4KB buffer ?

Normally multiple recvmsg() calls would be needed, but I do not see how
the nla_put_u32(skb, TCAA_ACT_COUNT, cb->args[1]) can always succeed.