Re: [Qemu-devel] [PATCH v12 6/8] mm: support reporting free page blocks

[Wang, Wei W]

On Tuesday, July 25, 2017 8:42 PM, hal Hocko wrote:
> On Tue 25-07-17 19:56:24, Wei Wang wrote:
> > On 07/25/2017 07:25 PM, Michal Hocko wrote:
> > >On Tue 25-07-17 17:32:00, Wei Wang wrote:
> > >>On 07/24/2017 05:00 PM, Michal Hocko wrote:
> > >>>On Wed 19-07-17 20:01:18, Wei Wang wrote:
> > >>>>On 07/19/2017 04:13 PM, Michal Hocko wrote:
> > >>>[...
> > We don't need to do the pfn walk in the guest kernel. When the API
> > reports, for example, a 2MB free page block, the API caller offers to
> > the hypervisor the base address of the page block, and size=2MB, to
> > the hypervisor.
> 
> So you want to skip pfn walks by regularly calling into the page allocator to
> update your bitmap. If that is the case then would an API that would allow you
> to update your bitmap via a callback be s sufficient? Something like
>       void walk_free_mem(int node, int min_order,
>                       void (*visit)(unsigned long pfn, unsigned long 
> nr_pages))
> 
> The function will call the given callback for each free memory block on the 
> given
> node starting from the given min_order. The callback will be strictly an 
> atomic
> and very light context. You can update your bitmap from there.

I would need to introduce more about the background here:
The hypervisor and the guest live in their own address space. The hypervisor's 
bitmap
isn't seen by the guest. I think we also wouldn't be able to give a callback 
function 
from the hypervisor to the guest in this case.

> 
> This would address my main concern that the allocator internals would get
> outside of the allocator proper. 

What issue would it have to expose the internal, for_each_zone()?
I think new code which would call it will also be strictly checked when they
are pushed to upstream.

Best,
Wei