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

[Aneesh Kumar K.V]

On 08/22/2018 10:46 PM, Aneesh Kumar K.V 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 at kernel.crashing.org>
> 

Fixes: bd5050e38aec3
("powerpc/mm/radix: Change pte relax sequence to handle nest MMU hang")

> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar at linux.ibm.com>
> ---
>   arch/powerpc/mm/pgtable-radix.c | 8 +++++---
>   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);
>   		/*
>