Subject: Re: [PATCH 2/6] macvtap: zerocopy: fix truesize underestimation

From: Jason Wang <[email-masked]>
Date: Mon, 16 Apr 2012 16:23:59 +0800

On 04/16/2012 03:14 PM, Michael S. Tsirkin wrote:
> On Mon, Apr 16, 2012 at 02:07:59PM +0800, Jason Wang wrote:
>> As the skb fragment were pinned/built from user pages, we should
>> account the page instead of length for truesize.
>>
>> Signed-off-by: Jason Wang<[email-masked]>
> I'm not sure this is right: the skb does *not* consume the
> whole page, userspace uses the rest of the page
> for other skbs. So we'll end up accounting for the
> same page twice.
> Eric, what's the right thing to do here in your opinion?

Or at very least, we need to do this in skb_copy_ubufs() as it allocate 
whole new pages.
>> ---
>>   drivers/net/macvtap.c |    6 ++++--
>>   1 files changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
>> index bd4a70d..7cb2684 100644
>> --- a/drivers/net/macvtap.c
>> +++ b/drivers/net/macvtap.c
>> @@ -519,6 +519,7 @@ static int zerocopy_sg_from_iovec(struct sk_buff *skb, const struct iovec *from,
>>   		struct page *page[MAX_SKB_FRAGS];
>>   		int num_pages;
>>   		unsigned long base;
>> +		unsigned long truesize;
>>
>>   		len = from->iov_len - offset;
>>   		if (!len) {
>> @@ -533,10 +534,11 @@ static int zerocopy_sg_from_iovec(struct sk_buff *skb, const struct iovec *from,
>>   		    (num_pages>  MAX_SKB_FRAGS - skb_shinfo(skb)->nr_frags))
>>   			/* put_page is in skb free */
>>   			return -EFAULT;
>> +		truesize = size * PAGE_SIZE;
>>   		skb->data_len += len;
>>   		skb->len += len;
>> -		skb->truesize += len;
>> -		atomic_add(len,&skb->sk->sk_wmem_alloc);
>> +		skb->truesize += truesize;
>> +		atomic_add(truesize,&skb->sk->sk_wmem_alloc);
>>   		while (len) {
>>   			int off = base&  ~PAGE_MASK;
>>   			int size = min_t(int, len, PAGE_SIZE - off);
> --
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