Re: [PATCH v2 4/5] mm: Introduce Hinted pages

[David Hildenbrand]

On 25.07.19 22:37, Alexander Duyck wrote:
> On Thu, 2019-07-25 at 20:32 +0200, David Hildenbrand wrote:
>> On 25.07.19 19:38, Alexander Duyck wrote:
>>> On Thu, 2019-07-25 at 18:48 +0200, David Hildenbrand wrote:
>>>> On 25.07.19 17:59, Alexander Duyck wrote:
>>>>> On Thu, Jul 25, 2019 at 1:53 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>>>>> On 24.07.19 19:03, Alexander Duyck wrote:
>>>>>>> From: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
>>>
>>> <snip>
>>>
>>>>>> Can't we reuse one of the traditional page flags for that, not used
>>>>>> along with buddy pages? E.g., PG_dirty: Pages that were not hinted yet
>>>>>> are dirty.
>>>>>
>>>>> Reusing something like the dirty bit would just be confusing in my
>>>>> opinion. In addition it looks like Xen has also re-purposed PG_dirty
>>>>> already for another purpose.
>>>>
>>>> You brought up waste page management. A dirty bit for unprocessed pages
>>>> fits perfectly in this context. Regarding XEN, as long as it's not used
>>>> along with buddy pages, no issue.
>>>
>>> I would rather not have to dirty all pages that aren't hinted. That starts
>>> to get too invasive. Ideally we only modify pages if we are hinting on
>>> them. That is why I said I didn't like the use of a dirty bit. What we
>>> want is more of a "guaranteed clean" bit.
>>
>> Not sure if that is too invasive, but fair enough.
>>
>>>> FWIW, I don't even thing PG_offline matches to what you are using it
>>>> here for. The pages are not logically offline. They were simply buddy
>>>> pages that were hinted. (I'd even prefer a separate page type for that
>>>> instead - if we cannot simply reuse one of the other flags)
>>>>
>>>> "Offline pages" that are not actually offline in the context of the
>>>> buddy is way more confusing.
>>>
>>> Right now offline and hinted are essentially the same thing since the
>>> effect is identical.
>>
>> No they are not the same thing. Regarding virtio-balloon: You are free
>> to reuse any hinted pages immediate. Offline pages (a.k.a. inflated) you
>> might not generally reuse before deflating.
> 
> Okay, so it sounds like your perspective is a bit different than mine. I
> was thinking of it from the perspective of the host OS where in either
> case the guest has set the page as MADV_DONTNEED. You are looking at it
> from the guest perspective where Offline means the guest cannot use it.
> 
>>> There may be cases in the future where that is not the case, but with the
>>> current patch set they both result in the pages being evicted from the
>>> guest.
>>>
>>>>> If anything I could probably look at seeing if the PG_private flags
>>>>> are available when a page is in the buddy allocator which I suspect
>>>>> they probably are since the only users I currently see appear to be
>>>>> SLOB and compound pages. Either that or maybe something like PG_head
>>>>> might make sense since once we start allocating them we are popping
>>>>> the head off of the boundary list.
>>>>
>>>> Would also be fine with me.
>>>
>>> Actually I may have found an even better bit if we are going with the
>>> "reporting" name. I could probably use "PG_uptodate" since it looks like
>>> most of its uses are related to filesystems. I will wait till I hear from
>>> Matthew on what bits would be available for use before I update things.
>>
>> Also fine with me. In the optimal case we (in my opinion)
>> a) Don't reuse PG_offline
>> b) Don't use another page type
> 
> That is fine. I just need to determine the exact flag to use then. I'll do
> some more research and wait to see if anyone else from MM comunity has
> input or suggestions on the page flag to be used. From what I can tell it
> looks like there are a bunch of flag bits that are unused as far as the
> buddy pages are concerned so I should have a few to choose from.

Right, and I would favor that - at least less hacking with the
kexec/kdump interface :)

You can then go ahead and add

PG_hinted or PG_reported = PG_*younameit* and properly document how it
is being used along with PageBuddy() only.

-- 

Thanks,

David / dhildenb