linux-kernel - Re: [PATCH v4 04/14] KVM: s390: pv: avoid stalls when making pages secure

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Date:   Tue, 31 Aug 2021 17:11:05 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc:     kvm@...r.kernel.org, cohuck@...hat.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,
        Ulrich.Weigand@...ibm.com
Subject: Re: [PATCH v4 04/14] KVM: s390: pv: avoid stalls when making pages
 secure



On 31.08.21 17:00, Claudio Imbrenda wrote:
> On Tue, 31 Aug 2021 16:32:24 +0200
> Christian Borntraeger <borntraeger@...ibm.com> wrote:
> 
>> On 18.08.21 15:26, Claudio Imbrenda 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.
>>>
>>> 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).
>>
>> To me it looks like
>> handle_pv_uvc does not handle EAGAIN but also calls into this code. Is this code
>> path ok or do we need to change something here?
> 
> EAGAIN will be propagated all the way to userspace, which will retry.
> 
> if the UVC fails, the page does not get unpinned, and the next attempt
> to run the UVC in the guest will trigger this same path.
> 
> if you don't like it, I can change handle_pv_uvc like this
> 
> 	if (rc == -EINVAL || rc == -EAGAIN)
> 
> which will save a trip to userspace

I think a comment would be good.
> 
>>
>>>
>>> Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
>>> Fixes: 214d9bbcd3a672 ("s390/mm: provide memory management functions for protected KVM guests")
>>> ---
>>>    arch/s390/kernel/uv.c | 29 +++++++++++++++++++++++------
>>>    1 file changed, 23 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) {
>>>    		/*
>>>    
>