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

[Dave Hansen]

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.