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

[Mike Rapoport]

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?

> 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.
 
> > 
> > I have no objection to renaming the balloon bit inside the kernel; I
> > think that's a wise idea.  I'm just not sure whether we should rename
> > the user balloon bit rather than adding a new bit.
> > 
> 
> Can we rename without breaking uapi?
> 
> -- 
> 
> Thanks,
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.