Subject: Re: [PATCH 1/3] powerpc/mm/radix: Update pte update sequence for pte clear case

From: Benjamin Herrenschmidt <[email-masked]>
Date: Thu, 09 Feb 2017 14:49:48 +1100

On Thu, 2017-02-09 at 08:28 +0530, Aneesh Kumar K.V wrote:
> In the kernel we do follow the below sequence in different code paths.
> pte = ptep_get_clear(ptep)
> ....
> set_pte_at(ptep, pte)
> 
> We do that for mremap, autonuma protection update and softdirty clearing. This
> implies our optimization to skip a tlb flush when clearing a pte update is
> not valid, because for DD1 system that followup set_pte_at will be done witout
> doing the required tlbflush. Fix that by always doing the dd1 style pte update
> irrespective of new_pte value. In a later patch we will optimize the application
> exit case.

What about my change to set_pte_at() ? We seem to be overwriting valid PTEs,
shouldn't we deal with that ?

Cheers,
Ben.

> Signed-off-by: Benjamin Herrenschmidt <[email-masked]>
> > Signed-off-by: Aneesh Kumar K.V <[email-masked]>
> ---
>  arch/powerpc/include/asm/book3s/64/radix.h | 12 +++---------
>  1 file changed, 3 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/book3s/64/radix.h b/arch/powerpc/include/asm/book3s/64/radix.h
> index b4d1302387a3..70a3cdcdbe47 100644
> --- a/arch/powerpc/include/asm/book3s/64/radix.h
> +++ b/arch/powerpc/include/asm/book3s/64/radix.h
> @@ -144,16 +144,10 @@ static inline unsigned long radix__pte_update(struct mm_struct *mm,
> >  		 * new value of pte
> >  		 */
> >  		new_pte = (old_pte | set) & ~clr;
> > -		/*
> > -		 * If we are trying to clear the pte, we can skip
> > -		 * the below sequence and batch the tlb flush. The
> > -		 * tlb flush batching is done by mmu gather code
> > -		 */
> > -		if (new_pte) {
> > -			asm volatile("ptesync" : : : "memory");
> > -			radix__flush_tlb_pte_p9_dd1(old_pte, mm, addr);
> > +		asm volatile("ptesync" : : : "memory");
> > +		radix__flush_tlb_pte_p9_dd1(old_pte, mm, addr);
> > +		if (new_pte)
> >  			__radix_pte_update(ptep, 0, new_pte);
> > -		}
> >  	} else
> >  		old_pte = __radix_pte_update(ptep, clr, set);
> >  	asm volatile("ptesync" : : : "memory");