Virtualization - Jun 2018 - [PATCH v34 2/4] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

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?

-- 
MST

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. This is the 
same for both cases.
For today's guests, usually there will be only one 4MB allocated and 
passed to get_from_free_page_list.

Best,
Wei

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?

> This is the same
> for both cases.
> For today's guests, usually there will be only one 4MB allocated and
passed
> to get_from_free_page_list.
> 
> Best,
> Wei
> 
>