Subject: Re: [PATCH] Fixed zero copy GSO without orphaning the fragments

From: Anton Nayshtut <[email-masked]>
Date: Tue, 20 May 2014 15:07:03 +0300

Hi Michael,

You're absolutely right.

We detected the actual corruption running MS HCK on earlier kernels,
before the 1fd819ec, so the patch was developed as a fix for this issue.

However, 1fd819ec fixes the corruption and now it's only an optimization
that re-enables the zero copy for this case.

We're collecting the numbers right now and will post them as soon as
possible.

Best Regards,
Anton

On 5/20/2014 2:50 PM, Michael S. Tsirkin wrote:
> On Tue, May 20, 2014 at 02:24:21PM +0300, Igor Royzis wrote:
>> Fix accessing GSO fragments memory (and a possible corruption therefore) after
>> reporting completion in a zero copy callback. The previous fix in the commit 1fd819ec
>> orphaned frags which eliminates zero copy advantages. The fix makes the completion
>> called after all the fragments were processed avoiding unnecessary orphaning/copying
>> from userspace.
>>
>> The GSO fragments corruption issue was observed in a typical QEMU/KVM VM setup that
>> hosts a Windows guest (since QEMU virtio-net Windows driver doesn't support GRO).
>> The fix has been verified by running the HCK OffloadLSO test.
>>
>> Signed-off-by: Igor Royzis <[email-masked]>
>> Signed-off-by: Anton Nayshtut <[email-masked]>
> 
> OK but with 1fd819ec there's no corruption, correct?
> So this patch is in fact an optimization?
> If true, I'd like to see some performance numbers please.
> 
> Thanks!
> 
>> ---
>>  include/linux/skbuff.h |    1 +
>>  net/core/skbuff.c      |   18 +++++++++++++-----
>>  2 files changed, 14 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
>> index 08074a8..8c49edc 100644
>> --- a/include/linux/skbuff.h
>> +++ b/include/linux/skbuff.h
>> @@ -287,6 +287,7 @@ struct skb_shared_info {
>>  	struct sk_buff	*frag_list;
>>  	struct skb_shared_hwtstamps hwtstamps;
>>  	__be32          ip6_frag_id;
>> +	struct sk_buff  *zcopy_src;
>>  
>>  	/*
>>  	 * Warning : all fields before dataref are cleared in __alloc_skb()
>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>> index 1b62343..6fa6342 100644
>> --- a/net/core/skbuff.c
>> +++ b/net/core/skbuff.c
>> @@ -610,14 +610,18 @@ EXPORT_SYMBOL(__kfree_skb);
>>   */
>>  void kfree_skb(struct sk_buff *skb)
>>  {
>> +	struct sk_buff *zcopy_src;
>>  	if (unlikely(!skb))
>>  		return;
>>  	if (likely(atomic_read(&skb->users) == 1))
>>  		smp_rmb();
>>  	else if (likely(!atomic_dec_and_test(&skb->users)))
>>  		return;
>> +	zcopy_src = skb_shinfo(skb)->zcopy_src;
>>  	trace_kfree_skb(skb, __builtin_return_address(0));
>>  	__kfree_skb(skb);
>> +	if (unlikely(zcopy_src))
>> +		kfree_skb(zcopy_src);
>>  }
>>  EXPORT_SYMBOL(kfree_skb);
>>  
>> @@ -662,14 +666,18 @@ EXPORT_SYMBOL(skb_tx_error);
>>   */
>>  void consume_skb(struct sk_buff *skb)
>>  {
>> +	struct sk_buff *zcopy_src;
>>  	if (unlikely(!skb))
>>  		return;
>>  	if (likely(atomic_read(&skb->users) == 1))
>>  		smp_rmb();
>>  	else if (likely(!atomic_dec_and_test(&skb->users)))
>>  		return;
>> +	zcopy_src = skb_shinfo(skb)->zcopy_src;
>>  	trace_consume_skb(skb);
>>  	__kfree_skb(skb);
>> +	if (unlikely(zcopy_src))
>> +		consume_skb(zcopy_src);
>>  }
>>  EXPORT_SYMBOL(consume_skb);
>>  
>> @@ -2867,7 +2875,6 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>>  	skb_frag_t *frag = skb_shinfo(head_skb)->frags;
>>  	unsigned int mss = skb_shinfo(head_skb)->gso_size;
>>  	unsigned int doffset = head_skb->data - skb_mac_header(head_skb);
>> -	struct sk_buff *frag_skb = head_skb;
>>  	unsigned int offset = doffset;
>>  	unsigned int tnl_hlen = skb_tnl_header_len(head_skb);
>>  	unsigned int headroom;
>> @@ -2913,7 +2920,6 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>>  			i = 0;
>>  			nfrags = skb_shinfo(list_skb)->nr_frags;
>>  			frag = skb_shinfo(list_skb)->frags;
>> -			frag_skb = list_skb;
>>  			pos += skb_headlen(list_skb);
>>  
>>  			while (pos < offset + len) {
>> @@ -2975,6 +2981,11 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>>  						 nskb->data - tnl_hlen,
>>  						 doffset + tnl_hlen);
>>  
>> +		if (skb_shinfo(head_skb)->tx_flags & SKBTX_DEV_ZEROCOPY) {
>> +			skb_shinfo(nskb)->zcopy_src = head_skb;
>> +			atomic_inc(&head_skb->users);
>> +		}
>> +
>>  		if (nskb->len == len + doffset)
>>  			goto perform_csum_check;
>>  
>> @@ -3001,7 +3012,6 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>>  				i = 0;
>>  				nfrags = skb_shinfo(list_skb)->nr_frags;
>>  				frag = skb_shinfo(list_skb)->frags;
>> -				frag_skb = list_skb;
>>  
>>  				BUG_ON(!nfrags);
>>  
>> @@ -3016,8 +3026,6 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>>  				goto err;
>>  			}
>>  
>> -			if (unlikely(skb_orphan_frags(frag_skb, GFP_ATOMIC)))
>> -				goto err;
>>  
>>  			*nskb_frag = *frag;
>>  			__skb_frag_ref(nskb_frag);
>> -- 
>> 1.7.9.5