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

[Wei Wang]

On 07/26/2017 06:24 PM, Michal Hocko wrote:
> On Wed 26-07-17 10:22:23, Wei Wang wrote:
> > 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).
> So what exactly prevents virtio-balloon from using the above proposed
> callback mechanism and export what is needed to the hypervisor?

I thought about it more. Probably we can use the callback function with 
a little change like this:

void walk_free_mem(void *opaque1, void (*visit)(void *opaque2, unsigned 
long pfn,
           unsigned long nr_pages))
{
    ...
    for_each_populated_zone(zone) {
                   for_each_migratetype_order(order, type) {
                        report_unused_page_block(zone, order, type, 
&page); // from patch 6
                        pfn = page_to_pfn(page);
                        visit(opaque1, pfn, 1 << order);
                    }
    }
}

The above function scans all the free list and directly sends each free 
page block to the
hypervisor via the virtio_balloon callback below. No need to implement a 
bitmap.

In virtio-balloon, we have the callback:
void *virtio_balloon_report_unused_pages(void *opaque,  unsigned long pfn,
unsigned long nr_pages)
{
    struct virtio_balloon *vb = (struct virtio_balloon *)opaque;
    ...put the free page block to the the ring of vb;
}


What do you think?


Best,
Wei