Re: [RFC for Linux] virtio_balloon: Add VIRTIO_BALLOON_F_THP_ORDER to handle THP spilt issue

[David Hildenbrand]

On 12.03.20 09:47, Michael S. Tsirkin wrote:
> On Thu, Mar 12, 2020 at 09:37:32AM +0100, David Hildenbrand wrote:
>> 2. You are essentially stealing THPs in the guest. So the fastest
>> mapping (THP in guest and host) is gone. The guest won't be able to make
>> use of THP where it previously was able to. I can imagine this implies a
>> performance degradation for some workloads. This needs a proper
>> performance evaluation.
> 
> I think the problem is more with the alloc_pages API.
> That gives you exactly the given order, and if there's
> a larger chunk available, it will split it up.
> 
> But for balloon - I suspect lots of other users,
> we do not want to stress the system but if a large
> chunk is available anyway, then we could handle
> that more optimally by getting it all in one go.
> 
> 
> So if we want to address this, IMHO this calls for a new API.
> Along the lines of
> 
> 	struct page *alloc_page_range(gfp_t gfp, unsigned int min_order,
> 					unsigned int max_order, unsigned int *order)
> 
> the idea would then be to return at a number of pages in the given
> range.
> 
> What do you think? Want to try implementing that?

You can just start with the highest order and decrement the order until
your allocation succeeds using alloc_pages(), which would be enough for
a first version. At least I don't see the immediate need for a new
kernel API.

-- 
Thanks,

David / dhildenb