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

[David Hildenbrand <david@xxxxxxxxxx>]

On 14.09.21 18:54, Claudio Imbrenda wrote:
> On Thu,  9 Sep 2021 18:22:46 +0200
> David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> > pte_map_lock() is sufficient.
>
> Can you explain the difference and why it is enough?

I didn't repeat the $subject:

"No need for pte_alloc_map_lock() if we know the pmd is present; 
pte_map_lock() is sufficient, because there isn't anything to allocate."

> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > ---
> >   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;


--
Thanks,

David / dhildenb