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

Michael S. Tsirkin
2026-04-13T20:32:19Z

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 <mst@redhat.com>
> > 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