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

On Mon, Dec 08, 2014 at 11:11:15AM +0000, David Vrabel wrote:

On 08/12/14 10:19, Luis Henriques wrote:

quoted

On Mon, Dec 01, 2014 at 09:55:24AM +0100, Stefan Bader wrote:

quoted

On 11.08.2014 19:32, Zoltan Kiss wrote:

quoted

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 <redacted>
Cc: Wei Liu <redacted>
Cc: Ian Campbell <redacted>
Cc: Paul Durrant <redacted>
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