[bug report] powerpc/mm/radix: Avoid flushing the PWC on every flush_tlb_range

[Dan Carpenter]

On Fri, Aug 11, 2017 at 03:16:08PM -0700, Tyrel Datwyler wrote:
> On 08/11/2017 01:15 PM, Dan Carpenter wrote:
> > Hello Benjamin Herrenschmidt,
> > 
> > This is a semi-automatic email about new static checker warnings.
> > 
> > The patch 424de9c6e3f8: "powerpc/mm/radix: Avoid flushing the PWC on 
> > every flush_tlb_range" from Jul 19, 2017, leads to the following 
> > Smatch complaint:
> > 
> > arch/powerpc/mm/tlb-radix.c:368 radix__flush_tlb_collapsed_pmd()
> > 	 error: we previously assumed 'mm' could be null (see line 362)
> > 
> > arch/powerpc/mm/tlb-radix.c
> >    361	
> >    362		pid = mm ? mm->context.id : 0;
> >                       ^^
> > Check for NULL.
> > 
> >    363		if (unlikely(pid == MMU_NO_CONTEXT))
> >    364			goto no_context;
> >    365	
> >    366		/* 4k page size, just blow the world */
> >    367		if (PAGE_SIZE == 0x1000) {
> >    368			radix__flush_all_mm(mm);
> >                                             ^^
> > Unchecked dereference.
> 
> Appears to be a false positive. MMU_NO_CONTEXT I believe is defined as "0". So, maybe it
> would be clearer that we take the goto branch if this line read:
> 
> 362		pid = mm ? mm->context.id : MMU_NO_CONTEXT;
> 

Ah...  This is because I'm compiling code for other arches in a "best
effort" way.  It doesn't pull in the right headers so it doesn't know
the value of MMU_NO_CONTEXT.  Otherwise it would read the code correctly
and not complain.

Sorry for that.

regards,
dan carpenter