Re: [Xen-devel] [PATCH] xen-netfront: Fix handling packets on compound pages with skb_linearize

Luis Henriques
2014-12-09T09:55:01Z

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