Re: [PATCH v34 2/4] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

[Wei Wang]

On 06/27/2018 10:41 AM, Michael S. Tsirkin wrote:
> On Wed, Jun 27, 2018 at 09:24:18AM +0800, Wei Wang wrote:
> > On 06/26/2018 09:34 PM, Michael S. Tsirkin wrote:
> > > On Tue, Jun 26, 2018 at 08:27:44PM +0800, Wei Wang wrote:
> > > > On 06/26/2018 11:56 AM, Michael S. Tsirkin wrote:
> > > > > On Tue, Jun 26, 2018 at 11:46:35AM +0800, Wei Wang wrote:
> > > > >
> > > > > > > > +	if (!arrays)
> > > > > > > > +		return NULL;
> > > > > > > > +
> > > > > > > > +	for (i = 0; i < max_array_num; i++) {
> > > > > > > So we are getting a ton of memory here just to free it up a bit later.
> > > > > > > Why doesn't get_from_free_page_list get the pages from free list for us?
> > > > > > > We could also avoid the 1st allocation then - just build a list
> > > > > > > of these.
> > > > > > That wouldn't be a good choice for us. If we check how the regular
> > > > > > allocation works, there are many many things we need to consider when pages
> > > > > > are allocated to users.
> > > > > > For example, we need to take care of the nr_free
> > > > > > counter, we need to check the watermark and perform the related actions.
> > > > > > Also the folks working on arch_alloc_page to monitor page allocation
> > > > > > activities would get a surprise..if page allocation is allowed to work in
> > > > > > this way.
> > > > > mm/ code is well positioned to handle all this correctly.
> > > > I'm afraid that would be a re-implementation of the alloc functions,
> > > A re-factoring - you can share code. The main difference is locking.
> > >
> > > > and
> > > > that would be much more complex than what we have. I think your idea of
> > > > passing a list of pages is better.
> > > >
> > > > Best,
> > > > Wei
> > > How much memory is this allocating anyway?
> > For every 2TB memory that the guest has, we allocate 4MB.
> Hmm I guess I'm missing something, I don't see it:
>
>
> +       max_entries = max_free_page_blocks(ARRAY_ALLOC_ORDER);
> +       entries_per_page = PAGE_SIZE / sizeof(__le64);
> +       entries_per_array = entries_per_page * (1 << ARRAY_ALLOC_ORDER);
> +       max_array_num = max_entries / entries_per_array +
> +                       !!(max_entries % entries_per_array);
>
> Looks like you always allocate the max number?

Yes. We allocated the max number and then free what's not used.
For example, a 16TB guest, we allocate Four 4MB buffers and pass the 4 
buffers to get_from_free_page_list. If it uses 3, then the remaining 1 
"4MB buffer" will end up being freed.

For today's guests, max_array_num is usually 1.

Best,
Wei