On Mon, Jan 30, 2012 at 04:43:33PM +0000, Ian Campbell wrote: > On Mon, 2012-01-30 at 16:23 +0000, Michael S. Tsirkin wrote: > > On Mon, Jan 30, 2012 at 03:51:53PM +0000, Ian Campbell wrote: > > > On Thu, 2012-01-26 at 13:11 +0000, Michael S. Tsirkin wrote: > > > > On Wed, Jan 25, 2012 at 12:27:14PM +0000, Ian Campbell wrote: > > > > > This prevents an issue where an ACK is delayed, a retransmit is queued (either > > > > > at the RPC or TCP level) and the ACK arrives before the retransmission hits the > > > > > wire. If this happens to an NFS WRITE RPC then the write() system call > > > > > completes and the userspace process can continue, potentially modifying data > > > > > referenced by the retransmission before the retransmission occurs. > > > > > > > > > > Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx> > > > > > Acked-by: Trond Myklebust <Trond.Myklebust@xxxxxxxxxx> > > > > > Cc: "David S. Miller" <davem@xxxxxxxxxxxxx> > > > > > Cc: Neil Brown <neilb@xxxxxxx> > > > > > Cc: "J. Bruce Fields" <bfields@xxxxxxxxxxxx> > > > > > Cc: linux-nfs@xxxxxxxxxxxxxxx > > > > > Cc: netdev@xxxxxxxxxxxxxxx > > > > > > > > This doesn't include either of the two options you proposed to address > > > > the sender blocked forever by receiver issue with bridged septups and > > > > endpoints such a tap device or a socket on the same box, > > > > does it? > > > > > > There was never any response to Bruce's question: > > > http://thread.gmane.org/gmane.linux.network/210873/focus=44849 > > > > > > Stupid question: Is it a requirement that you be safe against DOS by a > > > rogue process with a tap device? (And if so, does current code satisfy > > > that requirement?) > > > > > > IMHO the answer to both questions is no, there are plenty of ways a > > > rogue process with a tap device can wreak havoc. > > > > I thought the answer is an obviious yes :( > > What are these ways tap can wreak havoc? > > Can't they spoof traffic > and all sorts of things like that? > Hrm. I > suppose that the same as any peer on the network so we already handle > that sort of thing. Maybe that's a red herring then. Right. It typically does not include DOS on the sender :) > > > > > How about patching __skb_queue_tail to do a deep copy? > > > > That would seem to handle both tap and socket cases - > > > > any other ones left? > > > > > > Wouldn't that mean we were frequently (almost always) copying for lots > > > of other cases too? That would rather defeat the purpose of being able > > > to hand pages off to the network stack in a zero copy fashion. > > > > Yes but the case of an rpc connection to the same box > > is very rare I think, not worth optimizing for. > > But changing __skb_queue_tail doesn't only impact rpc connections to the > same box, does it? At least I can see plenty of callers of > __skb_queue_tail which don't look like they would want a copy to occur > -- plenty of drivers for one thing. > Perhaps in combination with a per-queue flag or per-socket flag to > enable it though it might work though? Right. I missed that. I'm guessing drivers don't hang on to skbs indefinitely. Still, copying is always safe - maybe the right thing to do is to add an __skb_queue_tail variant that does not copy, and gradually convert drivers that care to that API? > > > > If we do this, I think it would be beneficial to pass a flag > > > > to the destructor indicating that a deep copy was done: > > > > this would allow senders to detect that and adapt. > > > > > > If you were doing a deep copy anyway you might as well create a > > > completely new skb and release the old one, thereby causing the > > > destructors to fire in the normal way for it SKB. The copy wouldn't have > > > destructors because the pages would no longer be owned by the sender. > > > > > > Ian. > > > > What I mean is that page pin + deep copy might be more expensive > > than directly copying. So the owner of the original skb > > cares whether we did a deep copy or zero copy transmit worked. > > You mean so they can adaptively do a copy directly next time? > I think that would add a fair bit more complexity to what, as you point > out, is a fairly rare occurrence. > > Ian. For sunrpc yes but I was thinking about utilizing this mechanism for e.g. kvm in the future. It might be more common there. I agree this might be a future extension. -- MSt -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
