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

[Jason Wang]

On 2018å09æ07æ 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@xxxxxxxxxx>
> 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