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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     David Hildenbrand <david@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
Subject: Re: [PATCH 2/2] hv_balloon: do adjust_managed_page_count() when
 ballooning/un-ballooning
In-Reply-To: <9202aafa-f30e-4d96-72a9-3ccd083cc58c@redhat.com>
References: <20201202161245.2406143-1-vkuznets@redhat.com>
 <20201202161245.2406143-3-vkuznets@redhat.com>
 <9202aafa-f30e-4d96-72a9-3ccd083cc58c@redhat.com>
Date:   Thu, 03 Dec 2020 18:49:04 +0100
Message-ID: <871rg6ok4v.fsf@vitty.brq.redhat.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

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.

> LGTM
>
> Reviewed-by: David Hildenbrand <david@redhat.com>

Thanks!

-- 
Vitaly