linux-kernel - Re: [V2 PATCH 2/9] macvtap: zerocopy: fix truesize underestimation

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Message-ID: <1337180585.10741.6.camel@oc3660625478.ibm.com>
Date:	Wed, 16 May 2012 08:03:05 -0700
From:	Shirley Ma <mashirle@...ibm.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	eric.dumazet@...il.com, mst@...hat.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, ebiederm@...ssion.com,
	davem@...emloft.net
Subject: Re: [V2 PATCH 2/9] macvtap: zerocopy: fix truesize underestimation

On Wed, 2012-05-16 at 11:04 +0800, Jason Wang wrote:
> >> 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(). 

Then this would double count the size of "first" offset left from
previous copy, both skb->len and truesize.

Thanks
Shirley

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