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

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Date:	Thu, 18 Jul 2013 10:52:36 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 2/2] macvtap: do not zerocopy if iov needs more pages
 than MAX_SKB_FRAGS

On 07/17/2013 07:06 PM, Michael S. Tsirkin wrote:
> On Wed, Jul 17, 2013 at 06:32:11PM +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>
>> ---
>>  drivers/net/macvtap.c |   58 +++++++++++++++++++++++++++++++-----------------
>>  1 files changed, 37 insertions(+), 21 deletions(-)
>>
>> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
>> index a7c5654..e200f11 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 +766,19 @@ 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) {
>> +	if (m && m->msg_control && sock_flag(&q->sk, SOCK_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.
>>  		 */
> We don't linearize parts to make them fit, we simply
> fall back to data copy.
> So this comment no longer applies, right?

Right, I forget to remove them. Will do it in V2.
>> -		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;
>> +		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 +790,15 @@ 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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