Re: [PATCH net V2 1/2] tuntap: do not zerocopy if iov needs more pages than MAX_SKB_FRAGS

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jul 18, 2013 at 10:55:15AM +0800, Jason Wang wrote:
> We try to linearize part of the skb when the number of iov is greater than
> MAX_SKB_FRAGS. This is not enough since each single vector may occupy more than
> one pages, so zerocopy_sg_fromiovec() may still fail and may break the guest
> network.
> 
> Solve this problem by calculate the pages needed for iov before trying to do
> zerocopy and switch to use copy instead of zerocopy if it needs more than
> MAX_SKB_FRAGS.
> 
> This is done through introducing a new helper to count the pages for iov, and
> call uarg->callback() manually when switching from zerocopy to copy to notify
> vhost.
> 
> We can do further optimization on top.
> 
> The bug were introduced from commit 0690899b4d4501b3505be069b9a687e68ccbe15b
> (tun: experimental zero copy tx support)
> 
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Jason Wang <jasowang@redhat.com>


Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
> The patch is needed fot -stable.
> ---
>  drivers/net/tun.c |   62 ++++++++++++++++++++++++++++++++--------------------
>  1 files changed, 38 insertions(+), 24 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 5cdcf92..db690a3 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1035,6 +1035,29 @@ static int zerocopy_sg_from_iovec(struct sk_buff *skb, const struct iovec *from,
>  	return 0;
>  }
>  
> +static unsigned long iov_pages(const struct iovec *iv, int offset,
> +			       unsigned long nr_segs)
> +{
> +	unsigned long seg, base;
> +	int pages = 0, len, size;
> +
> +	while (nr_segs && (offset >= iv->iov_len)) {
> +		offset -= iv->iov_len;
> +		++iv;
> +		--nr_segs;
> +	}
> +
> +	for (seg = 0; seg < nr_segs; seg++) {
> +		base = (unsigned long)iv[seg].iov_base + offset;
> +		len = iv[seg].iov_len - offset;
> +		size = ((base & ~PAGE_MASK) + len + ~PAGE_MASK) >> PAGE_SHIFT;
> +		pages += size;
> +		offset = 0;
> +	}
> +
> +	return pages;
> +}
> +
>  /* Get packet from user space buffer */
>  static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>  			    void *msg_control, const struct iovec *iv,
> @@ -1082,32 +1105,18 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>  			return -EINVAL;
>  	}
>  
> -	if (msg_control)
> -		zerocopy = true;
> -
> -	if (zerocopy) {
> -		/* Userspace may produce vectors with count greater than
> -		 * MAX_SKB_FRAGS, so we need to linearize parts of the skb
> -		 * to let the rest of data to be fit in the frags.
> -		 */
> -		if (count > MAX_SKB_FRAGS) {
> -			copylen = iov_length(iv, count - MAX_SKB_FRAGS);
> -			if (copylen < offset)
> -				copylen = 0;
> -			else
> -				copylen -= offset;
> -		} else
> -				copylen = 0;
> -		/* There are 256 bytes to be copied in skb, so there is enough
> -		 * room for skb expand head in case it is used.
> +	if (msg_control) {
> +		/* There are 256 bytes to be copied in skb, so there is
> +		 * enough room for skb expand head in case it is used.
>  		 * The rest of the buffer is mapped from userspace.
>  		 */
> -		if (copylen < gso.hdr_len)
> -			copylen = gso.hdr_len;
> -		if (!copylen)
> -			copylen = GOODCOPY_LEN;
> +		copylen = gso.hdr_len ? gso.hdr_len : GOODCOPY_LEN;
>  		linear = copylen;
> -	} else {
> +		if (iov_pages(iv, offset + copylen, count) <= MAX_SKB_FRAGS)
> +			zerocopy = true;
> +	}
> +
> +	if (!zerocopy) {
>  		copylen = len;
>  		linear = gso.hdr_len;
>  	}
> @@ -1121,8 +1130,13 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>  
>  	if (zerocopy)
>  		err = zerocopy_sg_from_iovec(skb, iv, offset, count);
> -	else
> +	else {
>  		err = skb_copy_datagram_from_iovec(skb, 0, iv, offset, len);
> +		if (!err && msg_control) {
> +			struct ubuf_info *uarg = msg_control;
> +			uarg->callback(uarg, false);
> +		}
> +	}
>  
>  	if (err) {
>  		tun->dev->stats.rx_dropped++;
> -- 
> 1.7.1