[PATCHv5 09/17] mm/sparse: Check memmap alignment for compound_info_has_mask()

[Muchun Song]

> On Jan 28, 2026, at 21:54, Kiryl Shutsemau <kas at kernel.org> wrote:
> 
> If page->compound_info encodes a mask, it is expected that vmemmap to be
> naturally aligned to the maximum folio size.
> 
> Trigger a BUG() for CONFIG_DEBUG_VM=y or WARN() otherwise.
> 
> Signed-off-by: Kiryl Shutsemau <kas at kernel.org>
> Acked-by: Zi Yan <ziy at nvidia.com>
> ---
> mm/sparse.c | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
> 
> diff --git a/mm/sparse.c b/mm/sparse.c
> index b5b2b6f7041b..9c0f4015778c 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -600,6 +600,19 @@ void __init sparse_init(void)
> BUILD_BUG_ON(!is_power_of_2(sizeof(struct mem_section)));
> 	memblocks_present();
> 
> + 	if (compound_info_has_mask()) {
> + 		unsigned long alignment;
> + 		bool aligned;
> +
> + 		alignment = MAX_FOLIO_NR_PAGES * sizeof(struct page);
> + 		aligned = IS_ALIGNED((unsigned long) pfn_to_page(0), alignment);
> +
> + 		if (IS_ENABLED(CONFIG_DEBUG_VM))
> + 			BUG_ON(!aligned);
> + 		else
> + 			WARN_ON(!aligned);

Since you’ve fixed all the problematic architectures, I don’t believe
we’ll ever hit the WARN or BUG here anymore.

I think we can now simplify the code further and just use VM_BUG_ON:
if any architecture changes in the future, the misalignment will be
caught during testing, so we won’t need to worry about it at run-time.

> + 	}
> +
> 	pnum_begin = first_present_section_nr();
> 	nid_begin = sparse_early_nid(__nr_to_section(pnum_begin));
> 
> -- 
> 2.51.2
>