On Wed, May 18, 2011 at 09:02:23AM -0700, Shirley Ma wrote: > 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? I'm not sure it does. Look e.g. at tg3 - if expand_head fails packet gets dropped. No crash but unlikely to perform well :). > 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
