Re: [patch] mm, mempolicy: dummy slab_node return value for bugless kernels

[David Rientjes <rientjes@google.com>]

On Tue, 6 Mar 2012, Andrew Morton wrote:

> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -1611,6 +1611,7 @@ unsigned slab_node(struct mempolicy *policy)
> >  
> >  	default:
> >  		BUG();
> > +		return numa_node_id();
> >  	}
> >  }
> 
> Wait.  If the above code generated a warning then surely we get a *lot*
> of warnings!  I'd expect that a lot of code assumes that BUG() never
> returns?
> 

allyesconfig with CONFIG_BUG=n results in 50 such warnings tree wide, and 
this is the only one in mm/*.

> Also, does CONIG_BUG=n even make sense?  If we got here and we know
> that the kernel has malfunctioned, what point is there in pretending
> otherwise?  Odd.
> 

I don't suspect we'll be very popular if we try to remove it, I can see 
how it would be useful when BUG() is used when the problem isn't really 
fatal (to stop something like disk corruption), like the above case isn't. 
If policy->mode isn't one of MPOL_{BIND,INTERLEAVE,PREFERRED} then we'd 
want WARN_ON_ONCE() at best; someone either didn't test their patch or 
we've flipped a bit, but the kernel can run happily along using the local 
node for slab allocations while still notifying the user.

mm/mempolicy.c misuses BUG() in every case,  Not having the perfect NUMA 
optimizations is surely annoying, but let's not crash someone's kernel.

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