linux-kernel - Re: [PATCH kernel v4 7/7] virtio-balloon: tell host vm's unused page info

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Date:   Tue, 8 Nov 2016 10:30:30 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Li, Liang Z" <liang.z.li@...el.com>,
        "mst@...hat.com" <mst@...hat.com>
Cc:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "amit.shah@...hat.com" <amit.shah@...hat.com>,
        "quintela@...hat.com" <quintela@...hat.com>,
        "dgilbert@...hat.com" <dgilbert@...hat.com>,
        "qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "mgorman@...hsingularity.net" <mgorman@...hsingularity.net>,
        "cornelia.huck@...ibm.com" <cornelia.huck@...ibm.com>
Subject: Re: [PATCH kernel v4 7/7] virtio-balloon: tell host vm's unused page
 info

On 11/07/2016 09:50 PM, Li, Liang Z wrote:
> Sounds good.  Should we ignore some of the order-0 pages in step 4 if the bitmap is full?
> Or should retry to get a complete list of order-0 pages?

I think that's a pretty reasonable thing to do.

>>> It seems the benefit we get for this feature is not as big as that in fast
>> balloon inflating/deflating.
>>>>
>>>> You should not be using get_max_pfn().  Any patch set that continues
>>>> to use it is not likely to be using a proper algorithm.
>>>
>>> Do you have any suggestion about how to avoid it?
>>
>> Yes: get the pfns from the page free lists alone.  Don't derive
>> them from the pfn limits of the system or zones.
> 
> The ' get_max_pfn()' can be avoid in this patch, but I think we can't
> avoid it completely. We need it as a hint for allocating a proper
> size bitmap. No?

If you start with higher-order pages, you'll be unlikely to get anywhere
close to filling up a bitmap that was sized to hold all possible order-0
pages on the system.  Any use of max_pfn also means that you'll
completely mis-size bitmaps on sparse systems with large holes.

I think you should size it based on the size of the free lists, if anything.