netdev - Re: Re: WARNING: at kernel/softirq.c:159 _local_bh_enable

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Date:	Thu, 23 Feb 2012 15:21:06 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Francois Romieu <romieu@...zoreil.com>,
	Shreyas Bhatewara <sbhatewara@...are.com>,
	Jongman Heo <jongman.heo@...sung.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"Scott J. Goldman" <scottjg@...are.com>,
	VMware PV-Drivers <pv-drivers@...are.com>
Subject: Re: Re: WARNING: at kernel/softirq.c:159
 _local_bh_enable_ip+0x35/0x71()

Le jeudi 23 février 2012 à 14:22 +0100, Peter Zijlstra a écrit :
> On Thu, 2012-02-23 at 12:53 +0100, Eric Dumazet wrote:
> > Hmm, I am not sure we still need local_bh_disable()/local_bh_enable() in
> > kmap_skb_frag()/ kunmap_skb_frag() anymore after commit 3e4d3af501ccc
> > (mm: stack based kmap_atomic() )
> 
> The only thing to consider is keeping the total stack size under
> control, this is somewhat non-trivial since its non-obvious what all
> nests.
> 
> That said, you're probably right, and we do have a WARN in there
> (dependent on CONFIG_DEBUG_HIGHMEM) that yells if we exceed the
> available stack size.
> 
> The more 'interesting' exercise is determining a better upper bound on
> the stack size and updating kmap_types.h accordingly.
> 

I would say its the same logic than crypto : We might use at most two
contexts for SKB frags : USER or SOFTIRQ

We probably can remove KM_SKB_DATA_SOFTIRQ slot and use fact that this
kmap user can reuse existing USER/SOFTIRQ slots

> > diff --git a/net/core/kmap_skb.h b/net/core/kmap_skb.h
> > index 81e1ed7..06be5ee 100644
> > --- a/net/core/kmap_skb.h
> > +++ b/net/core/kmap_skb.h
> > @@ -2,18 +2,10 @@
> >  
> >  static inline void *kmap_skb_frag(const skb_frag_t *frag)
> >  {
> > -#ifdef CONFIG_HIGHMEM
> > -       BUG_ON(in_irq());
> > -
> > -       local_bh_disable();
> > -#endif
> >         return kmap_atomic(skb_frag_page(frag), KM_SKB_DATA_SOFTIRQ);
> >  }
> >  
> >  static inline void kunmap_skb_frag(void *vaddr)
> >  {
> >         kunmap_atomic(vaddr, KM_SKB_DATA_SOFTIRQ);
> > -#ifdef CONFIG_HIGHMEM
> > -       local_bh_enable();
> > -#endif
> >  } 
> 
> The nicer patch would of course be a patch that does:
> 
>  s/kunmap_skb_frag/kunmap_atomic/
>  s/kmap_skb_frag(\([^)]*\))/kmap_atomic(skb_frag_page(\1))/
> 
> There is no need to retain the KM_* argument and the 'helper' functions
> are quite pointless at that point.
> 
> 

Well, definitely a cleanup is possible, but probably not suitable for
3.3 and stable kernels ?


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