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

From: David Hildenbrand
Date: Thu Jul 25 2019 - 14:33:06 EST

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.

> 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



David / dhildenb