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Subject: Re: [PATCH resend RFC 7/9] s390/mm: no need for pte_alloc_map_lock()
 if we know the pmd is present
To:     Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc:     linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org,
        kvm@vger.kernel.org, linux-mm@kvack.org,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Niklas Schnelle <schnelle@linux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
        Ulrich Weigand <Ulrich.Weigand@de.ibm.com>
References: <20210909162248.14969-1-david@redhat.com>
 <20210909162248.14969-8-david@redhat.com>
 <20210914185449.42d7d5ca@p-imbrenda>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Message-ID: <5a572489-a6d6-aaf4-098a-a8059f9b12f0@redhat.com>
Date:   Tue, 14 Sep 2021 19:23:36 +0200
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On 14.09.21 18:54, Claudio Imbrenda wrote:
> 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?

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@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;
> 


-- 
Thanks,

David / dhildenb