linux-kernel - Re: [PATCH net V2 2/2] macvtap: do not zerocopy if iov needs more pages than MAX_SKB

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Date:	Tue, 30 Jul 2013 14:41:25 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net V2 2/2] macvtap: do not zerocopy if iov needs more
 pages than MAX_SKB_FRAGS

On Fri, Jul 19, 2013 at 08:03:51AM +0300, Michael S. Tsirkin wrote:
> On Thu, Jul 18, 2013 at 10:55:16AM +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.
> > 
> > This bug were introduced from b92946e2919134ebe2a4083e4302236295ea2a73
> > (macvtap: zerocopy: validate vectors before building skb).
> > 
> > Cc: Michael S. Tsirkin <mst@...hat.com>
> > Signed-off-by: Jason Wang <jasowang@...hat.com>
> 
> 
> Acked-by: Michael S. Tsirkin <mst@...hat.com>

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


> > ---
> > The patch is needed for -stable.
> > Changes from V1:
> > - remove the wrong comment
> > ---
> >  drivers/net/macvtap.c |   62 +++++++++++++++++++++++++++++-------------------
> >  1 files changed, 37 insertions(+), 25 deletions(-)
> > 
> > diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
> > index a7c5654..a98fb0e 100644
> > --- a/drivers/net/macvtap.c
> > +++ b/drivers/net/macvtap.c
> > @@ -698,6 +698,28 @@ static int macvtap_skb_to_vnet_hdr(const struct sk_buff *skb,
> >  	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 macvtap_get_user(struct macvtap_queue *q, struct msghdr *m,
> > @@ -744,31 +766,15 @@ static ssize_t macvtap_get_user(struct macvtap_queue *q, struct msghdr *m,
> >  	if (unlikely(count > UIO_MAXIOV))
> >  		goto err;
> >  
> > -	if (m && m->msg_control && sock_flag(&q->sk, SOCK_ZEROCOPY))
> > -		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 < vnet_hdr_len)
> > -				copylen = 0;
> > -			else
> > -				copylen -= vnet_hdr_len;
> > -		}
> > -		/* 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 buffer is mapped from userspace.
> > -		 */
> > -		if (copylen < vnet_hdr.hdr_len)
> > -			copylen = vnet_hdr.hdr_len;
> > -		if (!copylen)
> > -			copylen = GOODCOPY_LEN;
> > +	if (m && m->msg_control && sock_flag(&q->sk, SOCK_ZEROCOPY)) {
> > +		copylen = vnet_hdr.hdr_len ? vnet_hdr.hdr_len : GOODCOPY_LEN;
> >  		linear = copylen;
> > -	} else {
> > +		if (iov_pages(iv, vnet_hdr_len + copylen, count)
> > +		    <= MAX_SKB_FRAGS)
> > +			zerocopy = true;
> > +	}
> > +
> > +	if (!zerocopy) {
> >  		copylen = len;
> >  		linear = vnet_hdr.hdr_len;
> >  	}
> > @@ -780,9 +786,15 @@ static ssize_t macvtap_get_user(struct macvtap_queue *q, struct msghdr *m,
> >  
> >  	if (zerocopy)
> >  		err = zerocopy_sg_from_iovec(skb, iv, vnet_hdr_len, count);
> > -	else
> > +	else {
> >  		err = skb_copy_datagram_from_iovec(skb, 0, iv, vnet_hdr_len,
> >  						   len);
> > +		if (!err && m && m->msg_control) {
> > +			struct ubuf_info *uarg = m->msg_control;
> > +			uarg->callback(uarg, false);
> > +		}
> > +	}
> > +
> >  	if (err)
> >  		goto err_kfree;
> >  
> > -- 
> > 1.7.1
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