Re: [PATCH 2/2] powerpc/mm/hash: Only need the Nest MMU workaround for R -> RW transition

[Nicholas Piggin <npiggin@gmail.com>]

On Wed, 22 Aug 2018 22:46:05 +0530
"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> wrote:

> The Nest MMU workaround is only needed for RW upgrades. Avoid doing that
> for other pte updates.
> 
> We also avoid clearing the pte while marking it invalid. This is because other
> page table walk will find this pte none and can result in unexpected behaviour
> due to that. Instead we clear _PAGE_PRESENT and set the software pte bit
> _PAGE_INVALID. pte_present is already updated to check for bot the bits. This
> make sure page table walkers will find the pte present and things like
> pte_pfn(pte) returns the right value.
> 
> Based on the original patch from Benjamin Herrenschmidt <benh@kernel.crashing.org>
> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> ---
>  arch/powerpc/mm/pgtable-radix.c | 8 +++++---

This is powerpc/mm/radix, isn't it? Subject says hash.

Could we make this fix POWER9 only and use a RSV bit for it
rather than use up a SW bit? Other than that,

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>

>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/mm/pgtable-radix.c b/arch/powerpc/mm/pgtable-radix.c
> index 7be99fd9af15..c879979faa73 100644
> --- a/arch/powerpc/mm/pgtable-radix.c
> +++ b/arch/powerpc/mm/pgtable-radix.c
> @@ -1045,20 +1045,22 @@ void radix__ptep_set_access_flags(struct vm_area_struct *vma, pte_t *ptep,
>  	struct mm_struct *mm = vma->vm_mm;
>  	unsigned long set = pte_val(entry) & (_PAGE_DIRTY | _PAGE_ACCESSED |
>  					      _PAGE_RW | _PAGE_EXEC);
> +
> +	unsigned long change = pte_val(entry) ^ pte_val(*ptep);
>  	/*
>  	 * To avoid NMMU hang while relaxing access, we need mark
>  	 * the pte invalid in between.
>  	 */
> -	if (atomic_read(&mm->context.copros) > 0) {
> +	if ((change & _PAGE_RW) && atomic_read(&mm->context.copros) > 0) {
>  		unsigned long old_pte, new_pte;
>  
> -		old_pte = __radix_pte_update(ptep, ~0, 0);
> +		old_pte = __radix_pte_update(ptep, _PAGE_PRESENT, _PAGE_INVALID);
>  		/*
>  		 * new value of pte
>  		 */
>  		new_pte = old_pte | set;
>  		radix__flush_tlb_page_psize(mm, address, psize);
> -		__radix_pte_update(ptep, 0, new_pte);
> +		__radix_pte_update(ptep, _PAGE_INVALID, new_pte);
>  	} else {
>  		__radix_pte_update(ptep, 0, set);
>  		/*