Re: [Xen-devel] [PATCH] xen-netfront: pull on receive skb may need to happen earlier

Jan Beulich
2013-07-09T07:47:26Z

>>> On 08.07.13 at 17:22, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> On Mon, 2013-07-08 at 15:20 +0100, Jan Beulich wrote:
>
>> Okay, so with that done (see below) Dion is now seeing the
>> WARN_ON_ONCE(delta < len) in skb_try_coalesce() triggering. Of
>> course, with it having crashed before, it's hard to tell whether the
>> triggering now is an effect of the patch, or just got unmasked by it.
>
> Please note this warning is not the sign of an error.

Thanks for pointing this out.

> It can be triggered because network stack (IP + TCP) had to reallocate
> skb->head to hold all the headers.
>
> pskb_may_pull() doesn't change skb->truesize, for various reasons, so we
> can have a situation where the warning triggers.
>
> For example, it can happen when drivers use a really small skb->head,
> like 64 bytes, as its not big enough to hold ethernet+IP+TCP headers.

We know from the crashes prior to the patch here that this is
exactly what is happening.

Jan