Re: WARNING: at kernel/softirq.c:159 _local_bh_enable_ip+0x35/0x71()

Le jeudi 23 février 2012 à 11:42 +0100, Francois Romieu a écrit :
> (Heo, your mua broke threading)
> 
> Jongman Heo <jongman.heo@samsung.com> :
> > Francois Romieu<romieu@fr.zoreil.com>
> > > > Jongman Heo <jongman.heo@samsung.com> :
> > > > Following warning triggered with my VMware Linux guest, when NFS connection is requested from outside.
> > > 
> > > I'd try reverting 39d4a96fd7d2926e46151adbd18b810aeeea8ec0.
> [...]
> > I confirm that reverting the commit fixes the warning.
> 
> Shreyas, can you provide a proper fix for this bug (see
> http://www.spinics.net/lists/netdev/msg189554.html for details).
> 
> As far as I understand it you can not claim a fake transport layer header
> size for udp and blindly check the available buffer size through
> pskb_may_pull later. With a 32 bits HIGHMEM guest config (yuck...) it ends
> up enabling bh within a network device start_xmit context.
> 

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() )



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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.

> 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.


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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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On Thu, 2012-02-23 at 15:21 +0100, Eric Dumazet wrote:
> Well, definitely a cleanup is possible, but probably not suitable for
> 3.3 and stable kernels ? 

Nah, that'd be better suited for a development branch like :-)
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