Re: Is: SKB_MAX_LEN bites again. Was: Re: bug disabling guest interface

[ANNIE LI <annie.li@oracle.com>]

On 2013-3-9 10:57, Ian Campbell wrote:
>>> - change MAX_SKB_FRAGS to 19 to accommodate all guests
> Changing MAX_SKB_FRAGS is *not* an option upstream. This might be a
> useful local hack but we need to drop the idea as a long term fix.
>
>> Ugh. The negotiations between host and guest is probably the best
>> choice. The issues you are going to hit are that you might need
>> to redo the skbs to match what the frontend's max is.
> IMHO the right fix is for netback to coalesce as it copies from the
> frontend if it needs to do so, it is copying anyway so it should be
> cheap enough. I thought we had discussed this and someone was working on
> implementing it. If not Annie then perhaps it was Matt or Siva (both now
> CC'd)

Sorry, I have been working on some windows whql thing these days, so did 
not start it till now.
I did do some patch for netfront before(Sander mentioned later), which 
did some segment for large skbs in netfront. This is different from what 
you mentioned. I assume we need to do some work in both netback and 
netfront.

Thanks
Annie

>
> If necessary netback could even allocate a larger order head in order to
> accommodate very large packets, but I don't expect that to be required
> to fix the immediate issue we are seeing (but gives flexibility)
>
> This should get us past the immediate issue of the upstream change from
> 18->16  frags thing. Longer term the negotiation will allow us to avoid
> future incompatible changes in guest and host network stacks, as well as
> allowing frontends on other OSes (in particular Windows) to havea better
> chance of to DTRT.
>
>> Annie, Wei, Ian - were there some RFC patches floating around
>> for this?
>>
>>> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
>>> index 821c7f4..82de0f5 100644
>>> --- a/include/linux/skbuff.h
>>> +++ b/include/linux/skbuff.h
>>> @@ -143,8 +143,8 @@ struct sk_buff;
>>>    * Since GRO uses frags we allocate at least 16 regardless of page
>>>    * size.
>>>    */
>>> -#if (65536/PAGE_SIZE + 1)<  16
>>> -#define MAX_SKB_FRAGS 16UL
>>> +#if (65536/PAGE_SIZE + 1)<  19
>>> +#define MAX_SKB_FRAGS 19UL
>>>   #else
>>>   #define MAX_SKB_FRAGS (65536/PAGE_SIZE + 1)
>>>   #endif
>>>
>>>
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