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

[David Hildenbrand (Arm)]

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@xxxxxxxxxx>
> 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