Subject: Re: [RFC PATCH 0/7] Replace _PAGE_NUMA with PAGE_NONE protections

From: Mel Gorman <[email-masked]>
Date: Tue, 18 Nov 2014 17:08:25 +0000

On Tue, Nov 18, 2014 at 10:03:30PM +0530, Aneesh Kumar K.V wrote:
> > diff --git a/arch/powerpc/mm/copro_fault.c b/arch/powerpc/mm/copro_fault.c
> > index 5a236f0..46152aa 100644
> > --- a/arch/powerpc/mm/copro_fault.c
> > +++ b/arch/powerpc/mm/copro_fault.c
> > @@ -64,7 +64,12 @@ int copro_handle_mm_fault(struct mm_struct *mm, unsigned long ea,
> >  		if (!(vma->vm_flags & VM_WRITE))
> >  			goto out_unlock;
> >  	} else {
> > -		if (dsisr & DSISR_PROTFAULT)
> > +		/*
> > +		 * protfault should only happen due to us
> > +		 * mapping a region readonly temporarily. PROT_NONE
> > +		 * is also covered by the VMA check above.
> > +		 */
> > +		if (WARN_ON_ONCE(dsisr & DSISR_PROTFAULT))
> >  			goto out_unlock;
> >  		if (!(vma->vm_flags & (VM_READ | VM_EXEC)))
> >  			goto out_unlock;
> 
> 
> we should do that DSISR_PROTFAILT check after vma->vm_flags. It is not
> that we will not hit DSISR_PROTFAULT, what we want to ensure here is that
> we get a prot fault only for cases convered by that vma check. So
> everything should be taking the if (!(vma->vm_flags & (VM_READ |
> VM_EXEC))) branch if it is a protfault. If not we would like to know
> about that. And hence the idea of not using WARN_ON_ONCE. I was also not
> sure whether we want to enable that always. The reason for keeping that
> within CONFIG_DEBUG_VM is to make sure that nobody ends up depending on
> PROTFAULT outside the vma check convered. So expectations is that
> developers working on feature will run with DEBUG_VM enable and finds
> this warning. We don't expect to hit this otherwise.
> 

/me slaps self. It's clear now and updated accordingly. Thanks.

-- 
Mel Gorman
SUSE Labs