[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH] xen-netfront: Fix handling packets on compound pages with skb_linearize
On 08.12.2014 12:31, David Vrabel wrote: > On 08/12/14 11:21, Stefan Bader wrote: >> On 08.12.2014 12:11, David Vrabel wrote: >>> On 08/12/14 10:19, Luis Henriques wrote: >>>> On Mon, Dec 01, 2014 at 09:55:24AM +0100, Stefan Bader wrote: >>>>> On 11.08.2014 19:32, Zoltan Kiss wrote: >>>>>> There is a long known problem with the netfront/netback interface: if >>>>>> the guest >>>>>> tries to send a packet which constitues more than MAX_SKB_FRAGS + 1 ring >>>>>> slots, >>>>>> it gets dropped. The reason is that netback maps these slots to a frag >>>>>> in the >>>>>> frags array, which is limited by size. Having so many slots can occur >>>>>> since >>>>>> compound pages were introduced, as the ring protocol slice them up into >>>>>> individual (non-compound) page aligned slots. The theoretical worst case >>>>>> scenario looks like this (note, skbs are limited to 64 Kb here): >>>>>> linear buffer: at most PAGE_SIZE - 17 * 2 bytes, overlapping page >>>>>> boundary, >>>>>> using 2 slots >>>>>> first 15 frags: 1 + PAGE_SIZE + 1 bytes long, first and last bytes are >>>>>> at the >>>>>> end and the beginning of a page, therefore they use 3 * 15 = 45 slots >>>>>> last 2 frags: 1 + 1 bytes, overlapping page boundary, 2 * 2 = 4 slots >>>>>> Although I don't think this 51 slots skb can really happen, we need a >>>>>> solution >>>>>> which can deal with every scenario. In real life there is only a few >>>>>> slots >>>>>> overdue, but usually it causes the TCP stream to be blocked, as the >>>>>> retry will >>>>>> most likely have the same buffer layout. >>>>>> This patch solves this problem by linearizing the packet. This is not the >>>>>> fastest way, and it can fail much easier as it tries to allocate a big >>>>>> linear >>>>>> area for the whole packet, but probably easier by an order of magnitude >>>>>> than >>>>>> anything else. Probably this code path is not touched very frequently >>>>>> anyway. >>>>>> >>>>>> Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx> >>>>>> Cc: Wei Liu <wei.liu2@xxxxxxxxxx> >>>>>> Cc: Ian Campbell <Ian.Campbell@xxxxxxxxxx> >>>>>> Cc: Paul Durrant <paul.durrant@xxxxxxxxxx> >>>>>> Cc: netdev@xxxxxxxxxxxxxxx >>>>>> Cc: linux-kernel@xxxxxxxxxxxxxxx >>>>>> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx >>>>> >>>>> This does not seem to be marked explicitly as stable. Has someone already >>>>> asked >>>>> David Miller to put it on his stable queue? IMO it qualifies quite well >>>>> and the >>>>> actual change should be simple to pick/backport. >>>>> >>>> >>>> Thank you Stefan, I'm queuing this for the next 3.16 kernel release. >>> >>> Don't backport this yes. It's broken. It produces malformed requests >>> and netback will report a fatal error and stop all traffic on the VIF. >> >> Thanks David. Did this just come up? I don't remember seeing any report of >> the >> regression. :/ > > There's been a couple of reports on xen-devel recently with 3.17 > frontends and I've just repro'd it (by always forcing a skb_linearize() > in netfront). Ah ok. Found at least one now (plus your fixup proposal). Too long ago to remember for sure but I thought the change was tested by reporters. Could be that I only had tried the approach I was working on back then (which was more complicated trying to replace individual frags by aligned copies). :( And on our devel we have not yet switched to past 3.16. Hope I can squeeze in some testing there. -Stefan > > David > Attachment:
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