linux-kernel - Re: [PATCH v2 01/13] KVM: s390: pv: avoid stall notifications for some UVCs

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Message-ID: <87h7gd2y5c.fsf@redhat.com>
Date:   Thu, 29 Jul 2021 12:49:03 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org
Cc:     borntraeger@...ibm.com, frankja@...ux.ibm.com, thuth@...hat.com,
        pasic@...ux.ibm.com, david@...hat.com, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 01/13] KVM: s390: pv: avoid stall notifications for
 some UVCs

On Wed, Jul 28 2021, Claudio Imbrenda <imbrenda@...ux.ibm.com> wrote:

> Improve make_secure_pte to avoid stalls when the system is heavily
> overcommitted. This was especially problematic in kvm_s390_pv_unpack,
> because of the loop over all pages that needed unpacking.
>
> Also fix kvm_s390_pv_init_vm to avoid stalls when the system is heavily
> overcommitted.
>
> Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> ---
>  arch/s390/kernel/uv.c | 11 ++++++++---
>  arch/s390/kvm/pv.c    |  2 +-
>  2 files changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
> index aeb0a15bcbb7..fd0faa51c1bb 100644
> --- a/arch/s390/kernel/uv.c
> +++ b/arch/s390/kernel/uv.c
> @@ -196,11 +196,16 @@ static int make_secure_pte(pte_t *ptep, unsigned long addr,
>  	if (!page_ref_freeze(page, expected))
>  		return -EBUSY;
>  	set_bit(PG_arch_1, &page->flags);
> -	rc = uv_call(0, (u64)uvcb);
> +	rc = __uv_call(0, (u64)uvcb);
>  	page_ref_unfreeze(page, expected);
> -	/* Return -ENXIO if the page was not mapped, -EINVAL otherwise */
> -	if (rc)
> +	/*
> +	 * Return -ENXIO if the page was not mapped, -EINVAL for other errors.
> +	 * If busy or partially completed, return -EAGAIN.
> +	 */
> +	if (rc == 1)
>  		rc = uvcb->rc == 0x10a ? -ENXIO : -EINVAL;
> +	else if (rc > 1)
> +		rc = -EAGAIN;
>  	return rc;
>  }

Possibly dumb question: when does the call return > 1?
gmap_make_secure() will do a wait_on_page_writeback() for -EAGAIN, is
that always the right thing to do?