Re: [PATCH v5 04/14] KVM: s390: pv: avoid stalls when making pages secure

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Wed, 6 Oct 2021 17:54:00 +0200
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> Am 20.09.21 um 15:24 schrieb Claudio Imbrenda:
> > 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.
> > 
> > Due to the locks being held, it was not possible to simply replace
> > uv_call with uv_call_sched. A more complex approach was
> > needed, in which uv_call is replaced with __uv_call, which does not
> > loop. When the UVC needs to be executed again, -EAGAIN is returned, and
> > the caller (or its caller) will try again.
> > 
> > When -EAGAIN is returned, the path is the same as when the page is in
> > writeback (and the writeback check is also performed, which is
> > harmless).
> > 
> > Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> > Fixes: 214d9bbcd3a672 ("s390/mm: provide memory management functions for protected KVM guests")
> > ---
> >   arch/s390/kernel/uv.c     | 29 +++++++++++++++++++++++------
> >   arch/s390/kvm/intercept.c |  5 +++++
> >   2 files changed, 28 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
> > index aeb0a15bcbb7..68a8fbafcb9c 100644
> > --- a/arch/s390/kernel/uv.c
> > +++ b/arch/s390/kernel/uv.c
> > @@ -180,7 +180,7 @@ static int make_secure_pte(pte_t *ptep, unsigned long addr,
> >   {
> >   	pte_t entry = READ_ONCE(*ptep);
> >   	struct page *page;
> > -	int expected, rc = 0;
> > +	int expected, cc = 0;
> >   
> >   	if (!pte_present(entry))
> >   		return -ENXIO;
> > @@ -196,12 +196,25 @@ 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);
> > +	/*
> > +	 * If the UVC does not succeed or fail immediately, we don't want to
> > +	 * loop for long, or we might get stall notifications.
> > +	 * On the other hand, this is a complex scenario and we are holding a lot of
> > +	 * locks, so we can't easily sleep and reschedule. We try only once,
> > +	 * and if the UVC returned busy or partial completion, we return
> > +	 * -EAGAIN and we let the callers deal with it.
> > +	 */
> > +	cc = __uv_call(0, (u64)uvcb);
> >   	page_ref_unfreeze(page, expected);
> > -	/* Return -ENXIO if the page was not mapped, -EINVAL otherwise */
> > -	if (rc)
> > -		rc = uvcb->rc == 0x10a ? -ENXIO : -EINVAL;
> > -	return rc;
> > +	/*
> > +	 * Return -ENXIO if the page was not mapped, -EINVAL for other errors.
> > +	 * If busy or partially completed, return -EAGAIN.
> > +	 */
> > +	if (cc == UVC_CC_OK)
> > +		return 0;
> > +	else if (cc == UVC_CC_BUSY || cc == UVC_CC_PARTIAL)
> > +		return -EAGAIN;
> > +	return uvcb->rc == 0x10a ? -ENXIO : -EINVAL;
> >   }
> >   
> >   /*
> > @@ -254,6 +267,10 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
> >   	mmap_read_unlock(gmap->mm);
> >   
> >   	if (rc == -EAGAIN) {
> > +		/*
> > +		 * If we are here because the UVC returned busy or partial
> > +		 * completion, this is just a useless check, but it is safe.
> > +		 */
> >   		wait_on_page_writeback(page);
> >   	} else if (rc == -EBUSY) {
> >   		/*
> > diff --git a/arch/s390/kvm/intercept.c b/arch/s390/kvm/intercept.c
> > index 72b25b7cc6ae..47833ade4da5 100644
> > --- a/arch/s390/kvm/intercept.c
> > +++ b/arch/s390/kvm/intercept.c
> > @@ -516,6 +516,11 @@ static int handle_pv_uvc(struct kvm_vcpu *vcpu)
> >   	 */
> >   	if (rc == -EINVAL)
> >   		return 0;
> > +	/*
> > +	 * If we got -EAGAIN here, we simply return it. It will eventually
> > +	 * get propagated all the way to userspace, which should then try
> > +	 * again.
> > +	 */  
> 
> This cpoment is new over v4, right? Can this happen often? If not then this is ok
> otherwise we should consider your proposal of doing

yes, the comment is new. I would expect this to happen only when the
system is under load. in any case this is better than busy waiting on
the UVC

> 
> if (rc == -EINVAL || rc == -EAGAIN)
> 
> to reduce overhead.
> 
> Anyway,for both ways
> 
> Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
> 
> >   	return rc;
> >   }
> >   
> >