Re: [PATCH v7 25/31] mm: page_alloc: propagate PG_zeroed in split_large_buddy

[Gregory Price]

On Tue, May 12, 2026 at 05:07:20PM -0400, Michael S. Tsirkin wrote:
> When splitting a large buddy page, propagate the PG_zeroed flag
> to each sub-page before freeing it.  __free_pages_prepare clears
> all flags (including PG_zeroed), so the flag must be re-set on
> each fragment after the split.  This ensures that the buddy merge
> logic can see PG_zeroed on pages that were part of a larger
> zeroed block.
> 
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> Assisted-by: Claude:claude-opus-4-6
> Assisted-by: cursor-agent:GPT-5.4-xhigh

Reviewed-by: Gregory Price <gourry@xxxxxxxxxx>

> ---
>  mm/page_alloc.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 468e8bde7d34..ce43f5a3dbaa 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1523,6 +1523,7 @@ static void split_large_buddy(struct zone *zone, struct page *page,
>  			      unsigned long pfn, int order, fpi_t fpi)
>  {
>  	unsigned long end = pfn + (1 << order);
> +	bool zeroed = PageZeroed(page);
>  
>  	VM_WARN_ON_ONCE(!IS_ALIGNED(pfn, 1 << order));
>  	/* Caller removed page from freelist, buddy info cleared! */
> @@ -1534,6 +1535,8 @@ static void split_large_buddy(struct zone *zone, struct page *page,
>  	do {
>  		int mt = get_pfnblock_migratetype(page, pfn);
>  
> +		if (zeroed)
> +			__SetPageZeroed(page);
>  		__free_one_page(page, pfn, zone, order, mt, fpi);
>  		pfn += 1 << order;
>  		if (pfn == end)
> -- 
> MST
>