Messages in this thread | | | Date | Fri, 07 Mar 2014 16:58:06 +0100 | From | Thomas Graf <> | Subject | Re: [PATCH] openvswitch: Orphan frags before sending to userspace via Netlink to avoid guest stall |
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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;
and skb_zerocopy() will consume whatever tailroom is available first:
/* dont bother with small payloads */ if (len <= skb_tailroom(to)) { skb_copy_bits(from, 0, skb_put(to, len), len); return; }
I was planning to fix this while adding GSO support to the upcall as that is the moment when this bug would really surface.
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