Re: [PATCH v6 6/7] mm/mm_init: Fix uninitialized struct pages for ZONE_DEVICE

[Mike Rapoport <rppt@kernel.org>]

On Fri, Apr 24, 2026 at 10:55:46AM +0800, Muchun Song wrote:
> If DAX memory is hotplugged into an unoccupied subsection of an early
> section, section_activate() reuses the unoptimized boot memmap.
> However, compound_nr_pages() still assumes that vmemmap optimization is
> in effect and initializes only the reduced number of struct pages. As a
> result, the remaining tail struct pages are left uninitialized, which
> can later lead to unexpected behavior or crashes.
> 
> Fix this by treating early sections as unoptimized when calculating how
> many struct pages to initialize.
> 
> Fixes: 6fd3620b3428 ("mm/page_alloc: reuse tail struct pages for compound devmaps")
> Cc: stable@vger.kernel.org
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>

Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
>  mm/mm_init.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index cfc76953e249..bd466a3c10c8 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -1055,10 +1055,17 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
>   * of how the sparse_vmemmap internals handle compound pages in the lack
>   * of an altmap. See vmemmap_populate_compound_pages().
>   */
> -static inline unsigned long compound_nr_pages(struct vmem_altmap *altmap,
> +static inline unsigned long compound_nr_pages(unsigned long pfn,
> +					      struct vmem_altmap *altmap,
>  					      struct dev_pagemap *pgmap)
>  {
> -	if (!vmemmap_can_optimize(altmap, pgmap))
> +	/*
> +	 * If DAX memory is hot-plugged into an unoccupied subsection
> +	 * of an early section, the unoptimized boot memmap is reused.
> +	 * See section_activate().
> +	 */
> +	if (early_section(__pfn_to_section(pfn)) ||
> +	    !vmemmap_can_optimize(altmap, pgmap))
>  		return pgmap_vmemmap_nr(pgmap);
>  
>  	return VMEMMAP_RESERVE_NR * (PAGE_SIZE / sizeof(struct page));
> @@ -1128,7 +1135,7 @@ void __ref memmap_init_zone_device(struct zone *zone,
>  			continue;
>  
>  		memmap_init_compound(page, pfn, zone_idx, nid, pgmap,
> -				     compound_nr_pages(altmap, pgmap));
> +				     compound_nr_pages(pfn, altmap, pgmap));
>  	}
>  
>  	pageblock_migratetype_init_range(start_pfn, nr_pages, MIGRATE_MOVABLE);
> -- 
> 2.20.1
> 

-- 
Sincerely yours,
Mike.