linux-kernel - Re: [PATCH v9 11/18] s390/mm: KVM: pv: when tearing down, try to destroy protected pages

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Date:   Thu, 31 Mar 2022 15:34:42 +0200
From:   Janosch Frank <frankja@...ux.ibm.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org
Cc:     borntraeger@...ibm.com, thuth@...hat.com, pasic@...ux.ibm.com,
        david@...hat.com, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, scgl@...ux.ibm.com,
        mimu@...ux.ibm.com, nrb@...ux.ibm.com
Subject: Re: [PATCH v9 11/18] s390/mm: KVM: pv: when tearing down, try to
 destroy protected pages

On 3/30/22 14:25, Claudio Imbrenda wrote:
> When ptep_get_and_clear_full is called for a mm teardown, we will now
> attempt to destroy the secure pages. This will be faster than export.
> 
> In case it was not a teardown, or if for some reason the destroy page
> UVC failed, we try with an export page, like before.
> 
> Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> Acked-by: Janosch Frank <frankja@...ux.ibm.com>
> ---
>   arch/s390/include/asm/pgtable.h | 18 +++++++++++++++---
>   1 file changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
> index 23ca0d8e058a..72544a1b4a68 100644
> --- a/arch/s390/include/asm/pgtable.h
> +++ b/arch/s390/include/asm/pgtable.h
> @@ -1118,9 +1118,21 @@ static inline pte_t ptep_get_and_clear_full(struct mm_struct *mm,
>   	} else {
>   		res = ptep_xchg_lazy(mm, addr, ptep, __pte(_PAGE_INVALID));
>   	}
> -	/* At this point the reference through the mapping is still present */
> -	if (mm_is_protected(mm) && pte_present(res))
> -		uv_convert_owned_from_secure(pte_val(res) & PAGE_MASK);
> +	/* Nothing to do */
> +	if (!mm_is_protected(mm) || !pte_present(res))
> +		return res;
> +	/*
> +	 * At this point the reference through the mapping is still present.

That's the case because we zap ptes within a mm that's still existing, 
right? The mm will be deleted after we have unmapped the memory.


> +	 * The notifier should have destroyed all protected vCPUs at this
> +	 * point, so the destroy should be successful.
> +	 */
> +	if (full && !uv_destroy_owned_page(pte_val(res) & PAGE_MASK))
> +		return res;
> +	/*
> +	 * But if something went wrong and the pages could not be destroyed,
> +	 * the slower export is used as fallback instead.
> +	 */
> +	uv_convert_owned_from_secure(pte_val(res) & PAGE_MASK);
>   	return res;
>   }
>