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

[Wei Wang]

On 07/25/2017 10:53 PM, Michal Hocko wrote:
> On Tue 25-07-17 14:47:16, Wang, Wei W wrote:
> > 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.
> How did you plan to use your original API which export struct page array
> then?


That's where the virtio-balloon driver comes in. It uses a shared ring 
mechanism to
send the guest memory info to the hypervisor.

We didn't expose the struct page array from the guest to the hypervisor. 
For example, when
a 2MB free page block is reported from the free page list, the info put 
on the ring is just
(base address of the 2MB continuous memory, size=2M).


> > > 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()?
> zone is a MM internal concept. No code outside of the MM proper should
> really care about zones.

I think this is also what Andrew suggested in the previous discussion:
https://lkml.org/lkml/2017/3/16/951

Move the code to virtio-balloon and a little layering violation seems 
acceptable.


Best,
Wei