Re: [PATCH v10 19/37] mm: page_alloc: clear PG_zeroed on buddy merge if not both zero

[Lorenzo Stoakes <ljs@kernel.org>]

On Mon, Jun 08, 2026 at 04:38:08AM -0400, Michael S. Tsirkin wrote:
> When two buddy pages merge in __free_one_page(), preserve
> PG_zeroed on the merged page only if both buddies have the
> flag set.  Otherwise clear it.
>
> The merged page would inherit PG_zeroed, and a later __GFP_ZERO
> allocation would skip zeroing stale data in the non-zero half.
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> Reviewed-by: Gregory Price <gourry@gourry.net>
> Assisted-by: Claude:claude-opus-4-6
> Assisted-by: cursor-agent:GPT-5.4-xhigh
> ---
>  include/linux/page-flags.h |  1 +
>  mm/page_alloc.c            | 15 ++++++++++++++-
>  2 files changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 91f8ddb1d512..9365d59ac1d6 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -680,6 +680,7 @@ FOLIO_FLAG_FALSE(idle)
>   * uses this to skip redundant zeroing in post_alloc_hook().
>   */
>  __PAGEFLAG(Zeroed, zeroed, PF_NO_COMPOUND)
> +CLEARPAGEFLAG(Zeroed, zeroed, PF_NO_COMPOUND)
>  #define __PG_ZEROED (1UL << PG_zeroed)
>
>  /*
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index edfc83571985..a90bca5317c1 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -941,10 +941,14 @@ static inline void __free_one_page(struct page *page,
>  	unsigned long buddy_pfn = 0;
>  	unsigned long combined_pfn;
>  	struct page *buddy;
> +	bool buddy_zeroed;
> +	bool page_zeroed;
>  	bool to_tail;
>
>  	VM_BUG_ON(!zone_is_initialized(zone));
> -	VM_BUG_ON_PAGE(page->flags.f & PAGE_FLAGS_CHECK_AT_PREP, page);
> +	/* PG_zeroed (aliased to PG_private) is valid on free-list pages */
> +	VM_BUG_ON_PAGE(page->flags.f &

NIT: We don't add new VM_BUG_ON()'s, please use VM_WARN_ON().

> +		       (PAGE_FLAGS_CHECK_AT_PREP & ~__PG_ZEROED), page);
>
>  	VM_BUG_ON(migratetype == -1);
>  	VM_BUG_ON_PAGE(pfn & ((1 << order) - 1), page);
> @@ -979,6 +983,8 @@ static inline void __free_one_page(struct page *page,
>  				goto done_merging;
>  		}
>
> +		buddy_zeroed = PageZeroed(buddy);
> +
>  		/*
>  		 * Our buddy is free or it is CONFIG_DEBUG_PAGEALLOC guard page,
>  		 * merge with it and move up one order.
> @@ -997,10 +1003,17 @@ static inline void __free_one_page(struct page *page,
>  			change_pageblock_range(buddy, order, migratetype);
>  		}
>
> +		page_zeroed = PageZeroed(page);
> +		__ClearPageZeroed(page);
> +		__ClearPageZeroed(buddy);
> +
>  		combined_pfn = buddy_pfn & pfn;
>  		page = page + (combined_pfn - pfn);
>  		pfn = combined_pfn;
>  		order++;
> +
> +		if (page_zeroed && buddy_zeroed)
> +			__SetPageZeroed(page);
>  	}
>
>  done_merging:
> --
> MST
>