Re: [PATCH net-next 04/11] tuntap: simplify error handling in tun_build_skb()

[Jason Wang <jasowang@redhat.com>]

On 2018年09月07日 01:14, Michael S. Tsirkin wrote:
> On Thu, Sep 06, 2018 at 12:05:19PM +0800, Jason Wang wrote:
>> There's no need to duplicate page get logic in each action. So this
>> patch tries to get page and calculate the offset before processing XDP
>> actions, and undo them when meet errors (we don't care the performance
>> on errors). This will be used for factoring out XDP logic.
>>
>> Signed-off-by: Jason Wang <jasowang@redhat.com>
> I see some issues with this one.
>
>> ---
>>   drivers/net/tun.c | 37 ++++++++++++++++---------------------
>>   1 file changed, 16 insertions(+), 21 deletions(-)
>>
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index 372caf7d67d9..f8cdcfa392c3 100644
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -1642,7 +1642,7 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
>>   				     int len, int *skb_xdp)
>>   {
>>   	struct page_frag *alloc_frag = &current->task_frag;
>> -	struct sk_buff *skb;
>> +	struct sk_buff *skb = NULL;
>>   	struct bpf_prog *xdp_prog;
>>   	int buflen = SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
>>   	unsigned int delta = 0;
>> @@ -1668,6 +1668,9 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
>>   	if (copied != len)
>>   		return ERR_PTR(-EFAULT);
>>   
>> +	get_page(alloc_frag->page);
>> +	alloc_frag->offset += buflen;
>> +
> This adds an atomic op on XDP_DROP which is a data path
> operation for some workloads.

Yes, I have patch on top to amortize this, the idea is to have a very 
big refcount once after the frag was allocated and maintain a bias and 
decrease them all when allocating new frags.'

>
>>   	/* There's a small window that XDP may be set after the check
>>   	 * of xdp_prog above, this should be rare and for simplicity
>>   	 * we do XDP on skb in case the headroom is not enough.
>> @@ -1695,23 +1698,15 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
>>   
>>   		switch (act) {
>>   		case XDP_REDIRECT:
>> -			get_page(alloc_frag->page);
>> -			alloc_frag->offset += buflen;
>>   			err = xdp_do_redirect(tun->dev, &xdp, xdp_prog);
>>   			xdp_do_flush_map();
>>   			if (err)
>> -				goto err_redirect;
>> -			rcu_read_unlock();
>> -			local_bh_enable();
>> -			return NULL;
>> +				goto err_xdp;
>> +			goto out;
>>   		case XDP_TX:
>> -			get_page(alloc_frag->page);
>> -			alloc_frag->offset += buflen;
>>   			if (tun_xdp_tx(tun->dev, &xdp) < 0)
>> -				goto err_redirect;
>> -			rcu_read_unlock();
>> -			local_bh_enable();
>> -			return NULL;
>> +				goto err_xdp;
>> +			goto out;
>>   		case XDP_PASS:
>>   			delta = orig_data - xdp.data;
>>   			len = xdp.data_end - xdp.data;
>> @@ -1730,23 +1725,23 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
>>   	local_bh_enable();
>>   
>>   	skb = build_skb(buf, buflen);
>> -	if (!skb)
>> -		return ERR_PTR(-ENOMEM);
>> +	if (!skb) {
>> +		skb = ERR_PTR(-ENOMEM);
>> +		goto out;
> So goto out will skip put_page, and we did
> do get_page above. Seems wrong. You should
> goto err_skb or something like this.

Yes, looks like err_xdp.

>
>
>> +	}
>>   
>>   	skb_reserve(skb, pad - delta);
>>   	skb_put(skb, len);
>> -	get_page(alloc_frag->page);
>> -	alloc_frag->offset += buflen;
>>   
>>   	return skb;
>>   
>> -err_redirect:
>> -	put_page(alloc_frag->page);
>>   err_xdp:
>> +	alloc_frag->offset -= buflen;
>> +	put_page(alloc_frag->page);
>> +out:
> Out here isn't an error at all, is it?  You should not mix return and
> error handling IMHO.

If you mean the name, I can rename the label to "drop".

>
>
>
>>   	rcu_read_unlock();
>>   	local_bh_enable();
>> -	this_cpu_inc(tun->pcpu_stats->rx_dropped);
> Doesn't this break rx_dropped accounting?

Let me fix this.

Thanks

>> -	return NULL;
>> +	return skb;
>>   }
>>   
>>   /* Get packet from user space buffer */
>> -- 
>> 2.17.1