Re: [PATCH] mm: warn on unaligned compound zone device ranges

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 16 Oct 2026 21:13:34 +0800 Muchun Song <songmuchun@bytedance.com> wrote:

> A compound page of order N must start at a PFN aligned to 2^N pages.
> Therefore, a range used to initialize compound pages must start on a
> compound-page boundary and contain an integral number of compound pages.
> 
> Two recent Device DAX bugs were caused by ranges that violated this
> invariant, but neither was detected by memmap_init_zone_device().
> 
> Warn if the adjusted zone-device range violates this invariant so future
> violations are reported when the compound pages are initialized.
> 
> ...
>
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -1142,6 +1142,8 @@ void __ref memmap_init_zone_device(struct zone *zone,
>  	if (!nr_pages)
>  		return;
>  
> +	VM_WARN_ON(!IS_ALIGNED(start_pfn | nr_pages, pfns_per_compound));

Would VM_WARN_ON_ONCE() suffice?

> +
>  	/*
>  	 * Seed the reusable head-page template from the first real struct
>  	 * page. The normal page-init and refcount helpers must operate on
> 
> base-commit: 63562804f25cb5b6f5e4f5e3a9f0d1c3c6868596
> -- 
> 2.54.0