Re: [PATCHv4 07/14] mm/sparse: Check memmap alignment for compound_info_has_mask()

[Zi Yan]

On 21 Jan 2026, at 11:22, Kiryl Shutsemau wrote:

> If page->compound_info encodes a mask, it is expected that memmap to be
> naturally aligned to the maximum folio size.
>
> Add a warning if it is not.
>
> A warning is sufficient as MAX_FOLIO_ORDER is very rarely used, so the
> kernel is still likely to be functional if this strict check fails.
>
> Signed-off-by: Kiryl Shutsemau <kas@xxxxxxxxxx>
> ---
>  include/linux/mmzone.h | 1 +
>  mm/sparse.c            | 5 +++++
>  2 files changed, 6 insertions(+)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 390ce11b3765..7e4f69b9d760 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -91,6 +91,7 @@
>  #endif
>
>  #define MAX_FOLIO_NR_PAGES	(1UL << MAX_FOLIO_ORDER)
> +#define MAX_FOLIO_SIZE		(PAGE_SIZE << MAX_FOLIO_ORDER)
>
>  enum migratetype {
>  	MIGRATE_UNMOVABLE,
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 17c50a6415c2..5f41a3edcc24 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -600,6 +600,11 @@ void __init sparse_init(void)
>  	BUILD_BUG_ON(!is_power_of_2(sizeof(struct mem_section)));
>  	memblocks_present();
>
> +	if (compound_info_has_mask()) {
> +		WARN_ON(!IS_ALIGNED((unsigned long)pfn_to_page(0),
> +				    MAX_FOLIO_SIZE / sizeof(struct page)));
> +	}
> +

16GB is only possible in arm64 with 64KB base page. Would it be overkill
to align vmemmap to it unconditionally? Or how likely will this cause
false positive warning?

>  	pnum_begin = first_present_section_nr();
>  	nid_begin = sparse_early_nid(__nr_to_section(pnum_begin));
>

Anyway,

Acked-by: Zi Yan <ziy@xxxxxxxxxx>


Best Regards,
Yan, Zi