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

ANNIE LI
Tue, 12 Mar 2013 15:07:05 -0000

On 2013-3-12 20:18, Wei Liu wrote:
> On Tue, 2013-03-12 at 11:40 +0000, Ian Campbell wrote:
>> On Sun, 2013-03-10 at 19:18 +0000, Wei Liu wrote:
>>> On Sat, Mar 09, 2013 at 02:57:16AM +0000, 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)
>>>>
>>> As a short term fix, can we use skb_linearize() if
skb->nr_frags>>>> MAX_SKB_FRAGS?
>> No, because that would require changing the frontend, while this fix
>> needs to be in the backend if older guests are to continue working.
>>
>> You can''t use skb_linearize in netback as is because you would
first
>> need to be able to build the skb with nr_frags>= MAX_SKB_FRAGS in
order
>> to pass it to that function.
>>
> Yes, the idea is to define NETBK_MAX_SKB_FRAGS to some bigger number
> (say 20) to accommodate the possible maximum number of frags in
> frontend. The thing that truly matters it the skb->len, which should be
> <64K, nr_frags is not important.

I doubt this would work since you can not build out skb with nr_frags >=
MAX_SKB_FRAGS. See following code in skbuff.h,

#if (65536/PAGE_SIZE + 1) < 16
#define MAX_SKB_FRAGS 16UL
#else
#define MAX_SKB_FRAGS (65536/PAGE_SIZE + 1)
#endif

and every skb contains MAX_SKB_FRAGS frags,

struct skb_shared_info {
....
skb_frag_t      frags[MAX_SKB_FRAGS];
...
}

Coalescing frags before building skb could avoid this issue.

Thanks
Annie
>
> I wrote some code during weekend and it seemed to work, but I have not
> verify it extensively.
>
>
> Wei.
>
>
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