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

[Michal Hocko <mhocko@xxxxxxxxxx>]

[Cc Konstantin - the patch is 
http://lkml.kernel.org/r/20181114211704.6381-3-david@xxxxxxxxxx]

On Thu 15-11-18 10:21:13, David Hildenbrand wrote:
> 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)

I do not think any compilation could break. If the semantic of the flag
is preserved then everything should work as expected.
-- 
Michal Hocko
SUSE Labs

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