linux-kernel - Re: [PATCH net-next 06/11] tuntap: split out XDP logic

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Date:   Fri, 7 Sep 2018 11:29:34 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH net-next 06/11] tuntap: split out XDP logic



On 2018年09月07日 01:21, Michael S. Tsirkin wrote:
> On Thu, Sep 06, 2018 at 12:05:21PM +0800, Jason Wang wrote:
>> This patch split out XDP logic into a single function. This make it to
>> be reused by XDP batching path in the following patch.
>>
>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>> ---
>>   drivers/net/tun.c | 84 ++++++++++++++++++++++++++++-------------------
>>   1 file changed, 51 insertions(+), 33 deletions(-)
>>
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index 389aa0727cc6..21b125020b3b 100644
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -1635,6 +1635,44 @@ static bool tun_can_build_skb(struct tun_struct *tun, struct tun_file *tfile,
>>   	return true;
>>   }
>>   
>> +static u32 tun_do_xdp(struct tun_struct *tun,
>> +		      struct tun_file *tfile,
>> +		      struct bpf_prog *xdp_prog,
>> +		      struct xdp_buff *xdp,
>> +		      int *err)
>> +{
>> +	u32 act = bpf_prog_run_xdp(xdp_prog, xdp);
>> +
>> +	switch (act) {
>> +	case XDP_REDIRECT:
>> +		*err = xdp_do_redirect(tun->dev, xdp, xdp_prog);
>> +		xdp_do_flush_map();
>> +		if (*err)
>> +			break;
>> +		goto out;
>> +	case XDP_TX:
>> +		*err = tun_xdp_tx(tun->dev, xdp);
>> +		if (*err < 0)
>> +			break;
>> +		*err = 0;
>> +		goto out;
>> +	case XDP_PASS:
>> +		goto out;
> Do we need goto? why not just return?

I don't see any difference.

>
>> +	default:
>> +		bpf_warn_invalid_xdp_action(act);
>> +		/* fall through */
>> +	case XDP_ABORTED:
>> +		trace_xdp_exception(tun->dev, xdp_prog, act);
>> +		/* fall through */
>> +	case XDP_DROP:
>> +		break;
>> +	}
>> +
>> +	put_page(virt_to_head_page(xdp->data_hard_start));
> put here because caller does get_page :( Not pretty.
> I'd move this out to the caller.

Then we need a switch in the caller, not sure it's better.

>
>> +out:
>> +	return act;
> How about combining err and act? err is < 0 XDP_PASS is > 0.
> No need for pointers then.

Ok.

>
>> +}
>> +
>>   static struct sk_buff *tun_build_skb(struct tun_struct *tun,
>>   				     struct tun_file *tfile,
>>   				     struct iov_iter *from,
>> @@ -1645,10 +1683,10 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
>>   	struct sk_buff *skb = NULL;
>>   	struct bpf_prog *xdp_prog;
>>   	int buflen = SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
>> -	unsigned int delta = 0;
>>   	char *buf;
>>   	size_t copied;
>> -	int err, pad = TUN_RX_PAD;
>> +	int pad = TUN_RX_PAD;
>> +	int err = 0;
>>   
>>   	rcu_read_lock();
>>   	xdp_prog = rcu_dereference(tun->xdp_prog);
>> @@ -1685,9 +1723,8 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
>>   	local_bh_disable();
>>   	rcu_read_lock();
>>   	xdp_prog = rcu_dereference(tun->xdp_prog);
>> -	if (xdp_prog && !*skb_xdp) {
>> +	if (xdp_prog) {
>>   		struct xdp_buff xdp;
>> -		void *orig_data;
>>   		u32 act;
>>   
>>   		xdp.data_hard_start = buf;
>> @@ -1695,33 +1732,14 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
>>   		xdp_set_data_meta_invalid(&xdp);
>>   		xdp.data_end = xdp.data + len;
>>   		xdp.rxq = &tfile->xdp_rxq;
>> -		orig_data = xdp.data;
>> -		act = bpf_prog_run_xdp(xdp_prog, &xdp);
>> -
>> -		switch (act) {
>> -		case XDP_REDIRECT:
>> -			err = xdp_do_redirect(tun->dev, &xdp, xdp_prog);
>> -			xdp_do_flush_map();
>> -			if (err)
>> -				goto err_xdp;
>> -			goto out;
>> -		case XDP_TX:
>> -			if (tun_xdp_tx(tun->dev, &xdp) < 0)
>> -				goto err_xdp;
>> -			goto out;
>> -		case XDP_PASS:
>> -			delta = orig_data - xdp.data;
>> -			len = xdp.data_end - xdp.data;
>> -			break;
>> -		default:
>> -			bpf_warn_invalid_xdp_action(act);
>> -			/* fall through */
>> -		case XDP_ABORTED:
>> -			trace_xdp_exception(tun->dev, xdp_prog, act);
>> -			/* fall through */
>> -		case XDP_DROP:
>> +		act = tun_do_xdp(tun, tfile, xdp_prog, &xdp, &err);
>> +		if (err)
>>   			goto err_xdp;
>> -		}
>> +		if (act != XDP_PASS)
>> +			goto out;
> likely?

It depends on the XDP program, so I tend not to use it.

>
>> +
>> +		pad = xdp.data - xdp.data_hard_start;
>> +		len = xdp.data_end - xdp.data;
>>   	}
>>   	rcu_read_unlock();
>>   	local_bh_enable();
>> @@ -1729,18 +1747,18 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
>>   build:
>>   	skb = build_skb(buf, buflen);
>>   	if (!skb) {
>> +		put_page(alloc_frag->page);
>>   		skb = ERR_PTR(-ENOMEM);
>>   		goto out;
>>   	}
>>   
>> -	skb_reserve(skb, pad - delta);
>> +	skb_reserve(skb, pad);
>>   	skb_put(skb, len);
>>   
>>   	return skb;
>>   
>>   err_xdp:
>> -	alloc_frag->offset -= buflen;
>> -	put_page(alloc_frag->page);
>> +	this_cpu_inc(tun->pcpu_stats->rx_dropped);
>
> This fixes bug in previous patch which dropped it. OK :)

Yes, but let me move this to the buggy patch.

Thanks

>>   out:
>>   	rcu_read_unlock();
>>   	local_bh_enable();
>> -- 
>> 2.17.1