[ovs-dev] [runt-flows 5/8] datapath: Report memory allocation errors in flow_extract().

[Jesse Gross]

On Fri, Aug 27, 2010 at 9:47 AM, Ben Pfaff <blp at nicira.com> wrote:
> On Thu, Aug 26, 2010 at 02:56:04PM -0700, Jesse Gross wrote:
>> On Fri, Aug 13, 2010 at 10:55 AM, Ben Pfaff <blp at nicira.com> wrote:
>> > Until now flow_extract() has simply returned a bogus flow when memory
>> > allocation errors occurred.  This fixes the problem by propagating the
>> > error to the caller.
>> >
>> > Signed-off-by: Ben Pfaff <blp at nicira.com>
>>
>> Hmm, that's pretty nasty behavior that we had before.
>>
>> One general comment about this is that the behavior seems a little
>> asymmetrical for the different protocol types: why switch to this new
>> style for IP but not for ARP?  Why are the L4 protocols different from
>> L3?  Why can't we just do all of this at the beginning?  The answer to
>> all of these questions is (I assume) "IP options".  However, it took a
>> little more thought to convince myself that it is OK than seems
>> necessary.
>
> We could do it all at the beginning, if we are willing to pull 98 bytes
> of headers:
>
>        14      Ethernet
>        4       VLAN
>        60      max IP header
>        20      TCP/UDP/ICMP header
>        --
>        98
>
> The 64 bytes we currently pull covers any of the protocols we support as
> long as there are no IP options.  I was reluctant to increase the amount
> we pull.  But maybe it's not a big deal.  What do you think?

I don't think it is a good idea to increase the amount that we pull.
Xen puts the first 64 or 72 bytes (depending on the version) of the
packet in the linear data area and the rest is paged.  If we increase
the pull amount we'll end up reallocating and copying for every packet
larger than the value that Xen chooses.

I think that this just needs to be commented better.