Re: [virtio-dev] Re: [PATCH v9 0/8] stg mail -e --version=v9 \

From: David Hildenbrand
Date: Wed Sep 11 2019 - 05:51:09 EST

On 11.09.19 11:23, Michael S. Tsirkin wrote:
> On Tue, Sep 10, 2019 at 06:22:37PM +0200, David Hildenbrand wrote:
>> On 10.09.19 18:18, Dr. David Alan Gilbert wrote:
>>> * Alexander Duyck (alexander.duyck@xxxxxxxxx) wrote:
>>>> On Tue, Sep 10, 2019 at 7:47 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>>>>> On Tue 10-09-19 07:42:43, Alexander Duyck wrote:
>>>>>> On Tue, Sep 10, 2019 at 5:42 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>>>>>>> I wanted to review "mm: Introduce Reported pages" just realize that I
>>>>>>> have no clue on what is going on so returned to the cover and it didn't
>>>>>>> really help much. I am completely unfamiliar with virtio so please bear
>>>>>>> with me.
>>>>>>> On Sat 07-09-19 10:25:03, Alexander Duyck wrote:
>>>>>>> [...]
>>>>>>>> This series provides an asynchronous means of reporting to a hypervisor
>>>>>>>> that a guest page is no longer in use and can have the data associated
>>>>>>>> with it dropped. To do this I have implemented functionality that allows
>>>>>>>> for what I am referring to as unused page reporting
>>>>>>>> The functionality for this is fairly simple. When enabled it will allocate
>>>>>>>> statistics to track the number of reported pages in a given free area.
>>>>>>>> When the number of free pages exceeds this value plus a high water value,
>>>>>>>> currently 32, it will begin performing page reporting which consists of
>>>>>>>> pulling pages off of free list and placing them into a scatter list. The
>>>>>>>> scatterlist is then given to the page reporting device and it will perform
>>>>>>>> the required action to make the pages "reported", in the case of
>>>>>>>> virtio-balloon this results in the pages being madvised as MADV_DONTNEED
>>>>>>>> and as such they are forced out of the guest. After this they are placed
>>>>>>>> back on the free list,
>>>>>>> And here I am reallly lost because "forced out of the guest" makes me
>>>>>>> feel that those pages are no longer usable by the guest. So how come you
>>>>>>> can add them back to the free list. I suspect understanding this part
>>>>>>> will allow me to understand why we have to mark those pages and prevent
>>>>>>> merging.
>>>>>> Basically as the paragraph above mentions "forced out of the guest"
>>>>>> really is just the hypervisor calling MADV_DONTNEED on the page in
>>>>>> question. So the behavior is the same as any userspace application
>>>>>> that calls MADV_DONTNEED where the contents are no longer accessible
>>>>>> from userspace and attempting to access them will result in a fault
>>>>>> and the page being populated with a zero fill on-demand page, or a
>>>>>> copy of the file contents if the memory is file backed.
>>>>> As I've said I have no idea about virt so this doesn't really tell me
>>>>> much. Does that mean that if somebody allocates such a page and tries to
>>>>> access it then virt will handle a fault and bring it back?
>>>> Actually I am probably describing too much as the MADV_DONTNEED is the
>>>> hypervisor behavior in response to the virtio-balloon notification. A
>>>> more thorough explanation of it can be found by just running "man
>>>> madvise", probably best just to leave it at that since I am probably
>>>> confusing things by describing hypervisor behavior in a kernel patch
>>>> set.
>>>> For the most part all the page reporting really does is provide a way
>>>> to incrementally identify unused regions of memory in the buddy
>>>> allocator. That in turn is used by virtio-balloon in a polling thread
>>>> to report to the hypervisor what pages are not in use so that it can
>>>> make a decision on what to do with the pages now that it knows they
>>>> are unused.
>>>> All this is providing is just a report and it is optional if the
>>>> hypervisor will act on it or not. If the hypervisor takes some sort of
>>>> action on the page, then the expectation is that the hypervisor will
>>>> use some sort of mechanism such as a page fault to discover when the
>>>> page is used again.
>>> OK, that's interestingly different (but OK) from some other schemes that
>>> hav ebeen described which *require* the guest to somehow indicate the
>>> page is in use before starting to use the page again.
>> virtio-balloon also has a mode where the guest would not have to
>> indicate to the host before re-using a page. Only
>> VIRTIO_BALLOON_F_MUST_TELL_HOST enforces this. So it's not completely new.
> VIRTIO_BALLOON_F_MUST_TELL_HOST is a bit different.
> When it's not set, guest still must tell host about
> pages in use, it just can batch these notifications
> sending them possibly after page has been used.
> So even with VIRTIO_BALLOON_F_MUST_TELL_HOST off you don't
> skip the notification.

I don't think so

VIRTIO_BALLOON_F_MUST_TELL_HOST 0 /* Tell before reclaiming pages */

commit bf50e69f63d21091e525185c3ae761412be0ba72
Author: Dave Hansen <dave@xxxxxxxxxxxxxxxxxx>
Date: Thu Apr 7 10:43:25 2011 -0700

virtio balloon: kill tell-host-first logic


But, if the bit is _not_ set, we are under no obligation to
reverse the order; we're under no obligation to do _anything_.
As of now, qemu-kvm defines the bit, but doesn't set it.

Old code simply told the hypervisor afterwards, but only for little
performance gain. It is not strictly necessary.

> From hypervisor point of view, this feature is very much like adding
> page to the balloon and immediately taking it out of the balloon again,
> just doing it in one operation.
> The main difference is the contents of the page, which matters
> with poisoning: in that case hypervisor is expected to hand
> back page with the poisoning content. Not so with regular
> deflate where page contents is undefined.
> Well and also the new interface is optimized for large chunks
> of memory since we'll likely be dealing with such.
>>> Dave
>> --
>> Thanks,
>> David / dhildenb



David / dhildenb