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Re: [Xen-devel] [PATCH RFC 2/6] mm: convert PG_balloon to PG_offline
- To: Mike Rapoport <rppt@xxxxxxxxxxxxx>
- From: David Hildenbrand <david@xxxxxxxxxx>
- Date: Thu, 15 Nov 2018 10:21:13 +0100
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- Cc: Michal Hocko <mhocko@xxxxxxxx>, linux-doc@xxxxxxxxxxxxxxx, linux-mm@xxxxxxxxx, Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>, Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>, Jonathan Corbet <corbet@xxxxxxx>, "Michael S. Tsirkin" <mst@xxxxxxxxxx>, Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>, Matthew Wilcox <willy@xxxxxxxxxxxxx>, David Rientjes <rientjes@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, Alexey Dobriyan <adobriyan@xxxxxxxxx>, linux-pm@xxxxxxxxxxxxxxx, Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Vlastimil Babka <vbabka@xxxxxxx>, Christian Hansen <chansen3@xxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, Miles Chen <miles.chen@xxxxxxxxxxxx>, linux-fsdevel@xxxxxxxxxxxxxxx, devel@xxxxxxxxxxxxxxxxxxxxxx, Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>, "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
- Delivery-date: Thu, 15 Nov 2018 09:21:36 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
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
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