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Subject: Re: [PATCH 2/2] hv_balloon: do adjust_managed_page_count() when
 ballooning/un-ballooning
To:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        linux-hyperv@vger.kernel.org
Cc:     Wei Liu <wei.liu@kernel.org>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Michael Kelley <mikelley@microsoft.com>,
        Dexuan Cui <decui@microsoft.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
References: <20201202161245.2406143-1-vkuznets@redhat.com>
 <20201202161245.2406143-3-vkuznets@redhat.com>
 <9202aafa-f30e-4d96-72a9-3ccd083cc58c@redhat.com>
 <871rg6ok4v.fsf@vitty.brq.redhat.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <13524c28-dfec-dd21-8a45-216b161deb72@redhat.com>
Date:   Thu, 3 Dec 2020 18:49:39 +0100
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On 03.12.20 18:49, Vitaly Kuznetsov wrote:
> David Hildenbrand <david@redhat.com> writes:
> 
>> On 02.12.20 17:12, Vitaly Kuznetsov wrote:
>>> Unlike virtio_balloon/virtio_mem/xen balloon drivers, Hyper-V balloon driver
>>> does not adjust managed pages count when ballooning/un-ballooning and this leads
>>> to incorrect stats being reported, e.g. unexpected 'free' output.
>>>
>>> Note, the calculation in post_status() seems to remain correct: ballooned out
>>> pages are never 'available' and we manually add dm->num_pages_ballooned to
>>> 'commited'.
>>>
>>> Suggested-by: David Hildenbrand <david@redhat.com>
>>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>>> ---
>>>  drivers/hv/hv_balloon.c | 5 ++++-
>>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
>>> index da3b6bd2367c..8c471823a5af 100644
>>> --- a/drivers/hv/hv_balloon.c
>>> +++ b/drivers/hv/hv_balloon.c
>>> @@ -1198,6 +1198,7 @@ static void free_balloon_pages(struct hv_dynmem_device *dm,
>>>  		__ClearPageOffline(pg);
>>>  		__free_page(pg);
>>>  		dm->num_pages_ballooned--;
>>> +		adjust_managed_page_count(pg, 1);
>>>  	}
>>>  }
>>>  
>>> @@ -1238,8 +1239,10 @@ static unsigned int alloc_balloon_pages(struct hv_dynmem_device *dm,
>>>  			split_page(pg, get_order(alloc_unit << PAGE_SHIFT));
>>>  
>>>  		/* mark all pages offline */
>>> -		for (j = 0; j < alloc_unit; j++)
>>> +		for (j = 0; j < alloc_unit; j++) {
>>>  			__SetPageOffline(pg + j);
>>> +			adjust_managed_page_count(pg + j, -1);
>>> +		}
>>>  
>>>  		bl_resp->range_count++;
>>>  		bl_resp->range_array[i].finfo.start_page =
>>>
>>
>> I assume this has been properly tested such that it does not change the
>> system behavior regarding when/how HyperV decides to add/remove memory.
>>
> 
> I'm always reluctant to confirm 'proper testing' as no matter how small
> and 'obvious' the change is, regressions keep happening :-) But yes,
> this was tested on a Hyper-V host and 'stress' and I observed 'free'
> when the balloon was both inflated and deflated, values looked sane.

That;s what I wanted to hear ;)


-- 
Thanks,

David / dhildenb