linux-kernel - Re: [PATCH resend RFC 9/9] s390/mm: optimize reset_guest_reference

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Date:   Mon, 27 Sep 2021 19:02:07 +0200
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org, linux-mm@...ck.org,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Niklas Schnelle <schnelle@...ux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Ulrich Weigand <Ulrich.Weigand@...ibm.com>
Subject: Re: [PATCH resend RFC 9/9] s390/mm: optimize
 reset_guest_reference_bit()

On Thu,  9 Sep 2021 18:22:48 +0200
David Hildenbrand <david@...hat.com> wrote:

> We already optimize get_guest_storage_key() to assume that if we don't have
> a PTE table and don't have a huge page mapped that the storage key is 0.
> 
> Similarly, optimize reset_guest_reference_bit() to simply do nothing if
> there is no PTE table and no huge page mapped.
> 
> Signed-off-by: David Hildenbrand <david@...hat.com>

Reviewed-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>

> ---
>  arch/s390/mm/pgtable.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c
> index 534939a3eca5..50ab2fed3397 100644
> --- a/arch/s390/mm/pgtable.c
> +++ b/arch/s390/mm/pgtable.c
> @@ -901,13 +901,23 @@ int reset_guest_reference_bit(struct mm_struct *mm, unsigned long addr)
>  	pte_t *ptep;
>  	int cc = 0;
>  
> -	if (pmd_lookup(mm, addr, &pmdp))
> +	/*
> +	 * If we don't have a PTE table and if there is no huge page mapped,
> +	 * the storage key is 0 and there is nothing for us to do.
> +	 */
> +	switch (pmd_lookup(mm, addr, &pmdp)) {
> +	case -ENOENT:
> +		return 0;
> +	case 0:
> +		break;
> +	default:
>  		return -EFAULT;
> +	}
>  
>  	ptl = pmd_lock(mm, pmdp);
>  	if (!pmd_present(*pmdp)) {
>  		spin_unlock(ptl);
> -		return -EFAULT;
> +		return 0;
>  	}
>  
>  	if (pmd_large(*pmdp)) {