Subject: Re: [V2 PATCH 2/9] macvtap: zerocopy: fix truesize underestimation

From: Jason Wang <[email-masked]>
Date: Wed, 16 May 2012 11:04:14 +0800

On 05/16/2012 01:26 AM, Shirley Ma wrote:
> On Wed, 2012-05-02 at 11:41 +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]>
>> ---
>>   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;
> Here should be truesize = size * PAGE_SIZE - offset, right?
>

We get the whole user page, so need to account them all. Also this is 
aligned with skb_copy_ubufs().
>>                  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);
>>
>>