[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH] openvswitch: Orphan frags before sending to userspace via Netlink to avoid guest stall
On Fri, Mar 7, 2014 at 10:05 AM, Thomas Graf <tgraf@xxxxxxxxxx> wrote: > On 03/07/2014 06:19 PM, Pravin Shelar wrote: >> >> On Fri, Mar 7, 2014 at 7:58 AM, Thomas Graf <tgraf@xxxxxxxxxx> wrote: >>> >>> On 03/07/2014 05:46 AM, Pravin Shelar wrote: >>>> >>>> >>>> But I found bug in datapath user-space queue code. I am not sure how >>>> this can work with skb fragments and MMAP-netlink socket. >>>> Here is what happens, OVS allocates netlink skb and adds fragments to >>>> skb using skb_zero_copy(), then calls genlmsg_unicast(). >>>> But if netlink sock is mmped then netlink-send queues netlink >>>> allocated skb->head (linear data of skb) and ignore skb frags. >>>> >>>> Currently this is not problem with OVS vswitchd since it does not use >>>> netlink MMAP sockets. But if vswitchd stats using MMAP-netlink socket, >>>> it can break it. >>> >>> >>> >>> The secret is out ;-) >>> >>> I was very surprised too when I noticed that it worked. It's not just >>> OVS, it's nfqueue as well. The reason is that an netlink mmaped skb is >>> setup with a giant tailroom in netlink_ring_setup_skb(): >>> >>> skb->end = skb->tail + size; >>> >> For OVS use-case, the size is linear part of skb. so I think for >> mmap-netlink socket it will fail. > > > Could you rephrase? I'm not sure I understand correctly. > I meant OVS (queue_userspace_packet) passes skb_zerocopy_headlen() to genlmsg_new_unicast(). > The tailroom size equals to the configured frame payload size of > the ring buffer. So as long as the frame size chosen is large > enough to hold whatever pieces comes out of skb_gso_segment() we are > fine. That said, I agree that we should fix this properly before we > enable mmap on the OVS user space side. I see. So this work assuming that userspace sets nl_mmap_req.nm_frame_size large enough. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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