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

[David Vrabel <david.vrabel@xxxxxxxxxx>]

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

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