[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 08/12/14 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). David _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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