Re: [PATCH resend RFC 7/9] s390/mm: no need for pte_alloc_map_lock() if we know the pmd is present

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Thu,  9 Sep 2021 18:22:46 +0200
David Hildenbrand <david@redhat.com> wrote:

> pte_map_lock() is sufficient.

Can you explain the difference and why it is enough?

> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  arch/s390/mm/pgtable.c | 15 +++------------
>  1 file changed, 3 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c
> index 5fb409ff7842..4e77b8ebdcc5 100644
> --- a/arch/s390/mm/pgtable.c
> +++ b/arch/s390/mm/pgtable.c
> @@ -814,10 +814,7 @@ int set_guest_storage_key(struct mm_struct *mm, unsigned long addr,
>  	}
>  	spin_unlock(ptl);
>  
> -	ptep = pte_alloc_map_lock(mm, pmdp, addr, &ptl);
> -	if (unlikely(!ptep))
> -		return -EFAULT;
> -
> +	ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
>  	new = old = pgste_get_lock(ptep);
>  	pgste_val(new) &= ~(PGSTE_GR_BIT | PGSTE_GC_BIT |
>  			    PGSTE_ACC_BITS | PGSTE_FP_BIT);
> @@ -912,10 +909,7 @@ int reset_guest_reference_bit(struct mm_struct *mm, unsigned long addr)
>  	}
>  	spin_unlock(ptl);
>  
> -	ptep = pte_alloc_map_lock(mm, pmdp, addr, &ptl);
> -	if (unlikely(!ptep))
> -		return -EFAULT;
> -
> +	ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
>  	new = old = pgste_get_lock(ptep);
>  	/* Reset guest reference bit only */
>  	pgste_val(new) &= ~PGSTE_GR_BIT;
> @@ -977,10 +971,7 @@ int get_guest_storage_key(struct mm_struct *mm, unsigned long addr,
>  	}
>  	spin_unlock(ptl);
>  
> -	ptep = pte_alloc_map_lock(mm, pmdp, addr, &ptl);
> -	if (unlikely(!ptep))
> -		return -EFAULT;
> -
> +	ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
>  	pgste = pgste_get_lock(ptep);
>  	*key = (pgste_val(pgste) & (PGSTE_ACC_BITS | PGSTE_FP_BIT)) >> 56;
>  	paddr = pte_val(*ptep) & PAGE_MASK;