Re: [PATCH] xen-netfront: Fix handling packets on compound pages with skb_segment

On Mon, Aug 04, 2014 at 06:29:34PM +0100, Zoltan Kiss wrote:

On 31/07/14 21:25, David Miller wrote:

quoted

From: Zoltan Kiss <redacted>
Date: Wed, 30 Jul 2014 14:25:30 +0100

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 slicing up the skb itself with the help of
skb_segment, and calling xennet_start_xmit again on the resulting packets. It
also works with the theoretical worst case, where there is a 3 level recursion.
The good thing is that skb_segment only copies the header part, the frags will
be just referenced again.

Signed-off-by: Zoltan Kiss <redacted>

This is a really scary change :-)

I admit that :)

quoted

I definitely see some potential problem here.

First of all, even in cases where it might "work", such as TCP, you
are modifying the data stream.  The sizes are changing, the packet
counts are different, and all of this will have side effects such as
potentially harming TCP performance.

Secondly, for something like UDP you can't just split the packet up
like this, or for any other datagram protocol for that matter.

The netback/netfront interface currently only supports TSO and TSO6. That's
why I did the pktgen TCP patch

IMO if this approach is known to be broken in the future (say if we want
to support UFO) we'd better avoid it.

Wei.