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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 30 Jul 2013 14:41:13 +0300

On Fri, Jul 19, 2013 at 08:03:35AM +0300, Michael S. Tsirkin wrote:
> 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 <[email-masked]>
> > Signed-off-by: Jason Wang <[email-masked]>
> 
> 
> Acked-by: Michael S. Tsirkin <[email-masked]>

Was this patch dropped?
Any objections to merging it for 3.11 and stable?

> > ---
> > 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