Subject: Re: [PATCH RFC 1/9] mm: page_alloc: propagate PageReported flag across buddy splits

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 13 Apr 2026 16:32:07 -0400

On Mon, Apr 13, 2026 at 09:11:51PM +0200, David Hildenbrand (Arm) wrote:
> On 4/13/26 00:50, Michael S. Tsirkin wrote:
> > When a reported free page is split via expand() to satisfy a
> > smaller allocation, the sub-pages placed back on the free lists
> > lose the PageReported flag.  This means they will be unnecessarily
> > re-reported to the hypervisor in the next reporting cycle, wasting
> > work.
> > 
> > Propagate the PageReported flag to sub-pages during expand() so
> > that they are recognized as already-reported.
> > 
> > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > Assisted-by: Claude:claude-opus-4-6
> > ---
> >  mm/page_alloc.c | 17 ++++++++++++++---
> >  1 file changed, 14 insertions(+), 3 deletions(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 2d4b6f1a554e..edbb1edf463d 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1730,7 +1730,7 @@ struct page *__pageblock_pfn_to_page(unsigned long start_pfn,
> >   * -- nyc
> >   */
> >  static inline unsigned int expand(struct zone *zone, struct page *page, int low,
> > -				  int high, int migratetype)
> > +				  int high, int migratetype, bool reported)
> >  {
> >  	unsigned int size = 1 << high;
> >  	unsigned int nr_added = 0;
> > @@ -1752,6 +1752,15 @@ static inline unsigned int expand(struct zone *zone, struct page *page, int low,
> >  		__add_to_free_list(&page[size], zone, high, migratetype, false);
> >  		set_buddy_order(&page[size], high);
> >  		nr_added += size;
> > +
> > +		/*
> > +		 * The parent page has been reported to the host.  The
> > +		 * sub-pages are part of the same reported block, so mark
> > +		 * them reported too.  This avoids re-reporting pages that
> > +		 * the host already knows about.
> > +		 */
> 
> The comment is a bit excessive. I'd say you can drop it completely.
> 
> > +		if (reported)
> > +			__SetPageReported(&page[size]);
> >  	}
> >  
> >  	return nr_added;
> > @@ -1762,9 +1771,10 @@ static __always_inline void page_del_and_expand(struct zone *zone,
> >  						int high, int migratetype)
> >  {
> >  	int nr_pages = 1 << high;
> > +	bool was_reported = page_reported(page);
> >  
> >  	__del_page_from_free_list(page, zone, high, migratetype);
> > -	nr_pages -= expand(zone, page, low, high, migratetype);
> > +	nr_pages -= expand(zone, page, low, high, migratetype, was_reported);
> >  	account_freepages(zone, -nr_pages, migratetype);
> >  }
> >  
> > @@ -2322,7 +2332,8 @@ try_to_claim_block(struct zone *zone, struct page *page,
> >  
> >  		del_page_from_free_list(page, zone, current_order, block_type);
> >  		change_pageblock_range(page, current_order, start_type);
> > -		nr_added = expand(zone, page, order, current_order, start_type);
> > +		nr_added = expand(zone, page, order, current_order, start_type,
> > +				  false);
> 
> In MM land we started doing
> 
> /* reported= */false
> 
> 
> This raises a good question: how does buddy merging handle the reported
> flag?
> 
> -- 
> Cheers,
> 
> David

IIUC it doesn't: reported pages are never merged, if a page is merged it has
just entered buddy.

-- 
MST