Re: [PATCHv2 02/14] mm/sparse: Check memmap alignment

[Muchun Song <muchun.song@linux.dev>]

On 2025/12/18 23:09, Kiryl Shutsemau wrote:
> The upcoming changes in compound_head() require 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.

Different architectures default to 2 MB alignment (mainly to
enable huge mappings), which only accommodates folios up to
128 MB. Yet 1 GB huge pages are still fairly common, so
validating 16 GB (MAX_FOLIO_SIZE) alignment seems likely to
miss the most frequent case.

I’m concerned that this might plant a hidden time bomb: it
could detonate at any moment in later code, silently triggering
memory corruption or similar failures. Therefore, I don’t
think a WARNING is a good choice.

>
> Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
> ---
>   include/linux/mmzone.h | 1 +
>   mm/sparse.c            | 3 +++
>   2 files changed, 4 insertions(+)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 6cfede39570a..9f44dc760cdc 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..c5810ff7c6f7 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -600,6 +600,9 @@ void __init sparse_init(void)
>   	BUILD_BUG_ON(!is_power_of_2(sizeof(struct mem_section)));
>   	memblocks_present();
>   
> +	WARN_ON(!IS_ALIGNED((unsigned long)pfn_to_page(0),
> +			    MAX_FOLIO_SIZE / sizeof(struct page)));
> +
>   	pnum_begin = first_present_section_nr();
>   	nid_begin = sparse_early_nid(__nr_to_section(pnum_begin));
>