Re: [PATCH RFC 2/6] mm: convert PG_balloon to PG_offline

[David Hildenbrand]

On 15.11.18 03:07, Mike Rapoport wrote:
> On Wed, Nov 14, 2018 at 11:49:15PM +0100, David Hildenbrand wrote:
>> On 14.11.18 23:23, Matthew Wilcox wrote:
>>> On Wed, Nov 14, 2018 at 10:17:00PM +0100, David Hildenbrand wrote:
>>>> Rename PG_balloon to PG_offline. This is an indicator that the page is
>>>> logically offline, the content stale and that it should not be touched
>>>> (e.g. a hypervisor would have to allocate backing storage in order for the
>>>> guest to dump an unused page).  We can then e.g. exclude such pages from
>>>> dumps.
>>>>
>>>> In following patches, we will make use of this bit also in other balloon
>>>> drivers.  While at it, document PGTABLE.
>>>
>>> Thank you for documenting PGTABLE.  I didn't realise I also had this
>>> document to update when I added PGTABLE.
>>
>> Thank you for looking into this :)
>>
>>>
>>>> +++ b/Documentation/admin-guide/mm/pagemap.rst
>>>> @@ -78,6 +78,8 @@ number of times a page is mapped.
>>>>      23. BALLOON
>>>>      24. ZERO_PAGE
>>>>      25. IDLE
>>>> +    26. PGTABLE
>>>> +    27. OFFLINE
>>>
>>> So the offline *user* bit is new ... even though the *kernel* bit
>>> just renames the balloon bit.  I'm not sure how I feel about this.
>>> I'm going to think about it some more.  Could you share your decision
>>> process with us?
>>
>> BALLOON was/is documented as
>>
>> "23 - BALLOON
>>     balloon compaction page
>> "
>>
>> and only includes all virtio-ballon pages after the non-lru migration
>> feature has been implemented for ballooned pages. Since then, this flag
>> does basically no longer stands for what it actually was supposed to do.
> 
> Perhaps I missing something, but how the user should interpret "23" when he
> reads /proc/kpageflags?

Looking at the history in more detail:

commit 09316c09dde33aae14f34489d9e3d243ec0d5938
Author: Konstantin Khlebnikov <k.khlebnikov@xxxxxxxxxxx>
Date:   Thu Oct 9 15:29:32 2014 -0700

    mm/balloon_compaction: add vmstat counters and kpageflags bit

    Always mark pages with PageBalloon even if balloon compaction is
disabled
    and expose this mark in /proc/kpageflags as KPF_BALLOON.


So KPF_BALLOON was exposed when virtio-balloon pages were always marked
with PG_balloon. So the documentation is actually wrong ("balloon page"
vs. "balloon compaction page").

I have no idea who actually used that information. I suspect this was
just some debugging aid.

> 
>> To not break uapi I decided to not rename it but instead to add a new flag.
> 
> I've got a feeling that uapi was anyway changed for the BALLON flag
> meaning.

Yes. If we *replace* KPF_BALLOON by KPF_OFFLINE

a) Some applications might no longer compile (I guess that's ok)
b) Some old applications will treat KPF_OFFLINE like KPF_BALLOON (which
should at least for virtio-balloon usage until now be fine - it is just
more generic)

So I guess it's up to Maintainers/Matthew to decide :)

-- 

Thanks,

David / dhildenb