Messages in this thread | | | Date | Tue, 9 Dec 2014 09:54:53 +0000 | From | Luis Henriques <> | Subject | Re: [Xen-devel] [PATCH] xen-netfront: Fix handling packets on compound pages with skb_linearize |
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On Mon, Dec 08, 2014 at 11:11:15AM +0000, 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@citrix.com> > >>> Cc: Wei Liu <wei.liu2@citrix.com> > >>> Cc: Ian Campbell <Ian.Campbell@citrix.com> > >>> Cc: Paul Durrant <paul.durrant@citrix.com> > >>> Cc: netdev@vger.kernel.org > >>> Cc: linux-kernel@vger.kernel.org > >>> Cc: xen-devel@lists.xenproject.org > >> > >> 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. > > David
Ok, thank you. I've dropped it already.
Cheers, -- Luís
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