Re: [Qemu-devel] [PATCH v11 4/6] mm: function to offer a page block on the free list

[Christian Borntraeger]

On 06/20/2017 06:49 PM, David Hildenbrand wrote:
> On 20.06.2017 18:44, Rik van Riel wrote:
>> On Mon, 2017-06-12 at 07:10 -0700, Dave Hansen wrote:
>>
>>> The hypervisor is going to throw away the contents of these pages,
>>> right?  As soon as the spinlock is released, someone can allocate a
>>> page, and put good data in it.  What keeps the hypervisor from
>>> throwing
>>> away good data?
>>
>> That looks like it may be the wrong API, then?
>>
>> We already have hooks called arch_free_page and
>> arch_alloc_page in the VM, which are called when
>> pages are freed, and allocated, respectively.
>>
>> Nitesh Lal (on the CC list) is working on a way
>> to efficiently batch recently freed pages for
>> free page hinting to the hypervisor.
>>
>> If that is done efficiently enough (eg. with
>> MADV_FREE on the hypervisor side for lazy freeing,
>> and lazy later re-use of the pages), do we still
>> need the harder to use batch interface from this
>> patch?
>>
> David's opinion incoming:
> 
> No, I think proper free page hinting would be the optimum solution, if
> done right. This would avoid the batch interface and even turn
> virtio-balloon in some sense useless.
> 
Two reasons why I disagree:
- virtio-balloon is often used as memory hotplug. (e.g. libvirts current/max 
memory
uses virtio ballon)
- free page hinting will not allow to shrink the page cache of guests (like a 
ballooner does)