Re: [PATCH v1 05/29] mm/balloon_compaction: make PageOffline sticky until the page is freed

[David Hildenbrand]

On 30.06.25 18:01, Lorenzo Stoakes wrote:
> On Mon, Jun 30, 2025 at 02:59:46PM +0200, David Hildenbrand wrote:
> > Let the page freeing code handle clearing the page type.
>
> Why is this advantageous? We want to keep the page marked offline for longer?

Less code? ;)

I will add:

"Being able to identify balloon pages until actually freed is a 
requirement for upcoming movable_ops migration changes."

Note that the documentation is extended in patch #27 to mention that.

> > Acked-by: Zi Yan <ziy@xxxxxxxxxx>
> > Acked-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
>
> On assumption this UINT_MAX stuff is sane :)) I mean this is straightforward I
> guess:
> > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> > ---
> >   include/linux/balloon_compaction.h | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
> > index b9f19da37b089..bfc6e50bd004b 100644
> > --- a/include/linux/balloon_compaction.h
> > +++ b/include/linux/balloon_compaction.h
> > @@ -140,7 +140,7 @@ static inline void balloon_page_finalize(struct page *page)
> >   		__ClearPageMovable(page);
> >   		set_page_private(page, 0);
> >   	}
> > -	__ClearPageOffline(page);
> > +	/* PageOffline is sticky until the page is freed to the buddy. */
>
> OK so we are relying on this UINT_MAX thing in free_pages_prepare() to handle this.

Yes. Resetting the page_type -> _mapcount to the initial value -1.

--
Cheers,

David / dhildenb