Re: [PATCH v25 QEMU 3/3] virtio-balloon: Replace free page hinting references to 'report' with 'hint'

[David Hildenbrand <david@redhat.com>]

On 24.06.20 17:37, Michael S. Tsirkin wrote:
> On Wed, Jun 24, 2020 at 05:28:59PM +0200, David Hildenbrand wrote:
>>> So at the high level the idea was simple, we just clear the dirty bit
>>> when page is hinted, unless we sent a new command since. Implementation
>>> was reviewed by migration maintainers. If there's a consensus the code
>>> is written so badly we can't maintain it, maybe we should remove it.
>>> Which parts are unmaintainable in your eyes - migration or virtio ones?
>>
>> QEMU implementation without a propert virtio specification. I hope that
>> we can *at least* finally document the expected behavior. Alex gave it a
>> shot, and I was hoping that Wei could jump in to clarify, help move this
>> forward ... after all he implemented (+designed?) the feature and the
>> virtio interface.
>>
>>> Or maybe it's the general thing that interface was never specced
>>> properly.
>>
>> Yes, a spec would be definitely a good starter ...
>>
>> [...]
>>
>>>>
>>>> 1. If migration fails during RAM precopy, the guest will never receive a
>>>> DONE notification. Probably easy to fix.
>>>>
>>>> 2. Unclear semantics. Alex tried to document what the actual semantics
>>>> of hinted pages are.
>>>
>>> I'll reply to that now.
>>>
>>>> Assume the following in the guest to a previously
>>>> hinted page
>>>>
>>>> /* page was hinted and is reused now */
>>>> if (page[x] != Y)
>>>> 	page[x] == Y;
>>>> /* migration ends, we now run on the destination */
>>>> BUG_ON(page[x] != Y);
>>>> /* BUG, because the content chan
>>>
>>> The assumption hinting makes is that data in page is writtent to before it's used.
>>>
>>>
>>>> A guest can observe that. And that could be a random driver that just
>>>> allocated a page.
>>>>
>>>> (I *assume* in Linux we might catch that using kasan, but I am not 100%
>>>> sure, also, the actual semantics to document are unclear - e.g., for
>>>> other guests)
>>>
>>> I think it's basically simple: hinting means it's ok to
>>> fill page with trash unless it has been modified since the command
>>> ID supplied.
>>
>> Yeah, I quite dislike the semantics, especially, as they are different
>> to well-know semantics as e.g., represent in MADV_FREE. Getting changed
>> content when reading is really weird. But it seemed to be easier to
>> implement (low hanging fruit) and nobody complained back then. Well, now
>> we are stuck with it.
>>
>> [..]
> 
> The difference with MADV_FREE is
> - asynchronous (using cmd id to synchronize)
> - zero not guaranteed
> 
> right?

*looking into man page*, yes, when reading you either get the old
content or zero.

(I remember that a re-read also makes the content stable, but looks like
you really have to write to a page)

We should most probably do what Alex suggested and initialize pages (at
least write a single byte) when leaking them from the shrinker in the
guest while hinting is active, such that the content is stable for
anybody to allocate and reuse a page.

-- 
Thanks,

David / dhildenb