On Wed, 2011-05-18 at 07:38 -0700, Shirley Ma wrote: > On Wed, 2011-05-18 at 13:40 +0200, MichaÅ MirosÅaw wrote: > > >> >> Not more other restrictions, skb clone is OK. > pskb_expand_head() > > looks > > >> >> OK to me from code review. > > >> > Hmm. pskb_expand_head calls skb_release_data while keeping > > >> > references to pages. How is that ok? What do I miss? > > >> It's making copy of the skb_shinfo earlier, so the pages refcount > > >> stays the same. > > > Exactly. But the callback is invoked so the guest thinks it's ok > to > > > change this memory. If it does a corrupted packet will be sent > out. > > > > Hmm. I tool a quick look at skb_clone(), and it looks like this > > sequence will break this scheme: > > > > skb2 = skb_clone(skb...); > > kfree_skb(skb) or pskb_expand_head(skb); /* callback called */ > > [use skb2, pages still referenced] > > kfree_skb(skb); /* callback called again */ > > > > This sequence is common in bridge, might be in other places. > > > > Maybe this ubuf thing should just track clones? This will make it > work > > on all devices then. > > The callback was only invoked when last reference of skb was gone. > skb_clone does increase skb refcnt. I tested tcpdump on lower device, > it > worked. > > For the sequence of: > > skb_clone -> last refcnt + 1 > kfree_skb() or pskb_expand_head -> callback not called > kfree_skb() -> callback called > > I will check page refcount to see whether it's balanced. The page refcounts are balanced too. In macvtap/vhost Real NIC zerocopy case, it always goes to fastpath in pskb_expand_head, so I didn't hit any issue. But rethinking about pskb_expand_head(), it calls skb_release_data() to free old skb head when it's not in the fastpath (pskb_expand_head is not the last reference of this skb); And it's impossible to track which skb head (old one or new one) will be the last one to free. So better to return error for zero-copy skbs when not using fastpath. Does it make sense? Besides this, any other issue? Thanks Shirley -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
