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

From: David Vrabel <[email-masked]>
Date: Mon, 8 Dec 2014 11:11:15 +0000

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 <[email-masked]>
>>> Cc: Wei Liu <[email-masked]>
>>> Cc: Ian Campbell <[email-masked]>
>>> Cc: Paul Durrant <[email-masked]>
>>> Cc: [email-masked]
>>> Cc: [email-masked]
>>> Cc: [email-masked]
>>
>> 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