Re: [PATCH v1] mm/memblock: Correct totalram_pages accounting with KMSAN

[David Hildenbrand <david@redhat.com>]

On 17.09.25 14:32, Alexander Potapenko wrote:
> When KMSAN is enabled, `kmsan_memblock_free_pages()` can hold back pages
> for metadata instead of returning them to the early allocator. The callers,
> however, would unconditionally increment `totalram_pages`, assuming the
> pages were always freed. This resulted in an incorrect calculation of the
> total available RAM, causing the kernel to believe it had more memory than
> it actually did.
> 
> This patch refactors `memblock_free_pages()` to return the number of pages
> it successfully frees. If KMSAN stashes the pages, the function now
> returns 0; otherwise, it returns the number of pages in the block.
> 
> The callers in `memblock.c` have been updated to use this return value,
> ensuring that `totalram_pages` is incremented only by the number of pages
> actually returned to the allocator. This corrects the total RAM accounting
> when KMSAN is active.
> 
> Cc: Aleksandr Nogikh <nogikh@google.com>
> Fixes: 3c2065098260 ("init: kmsan: call KMSAN initialization routines")
> Signed-off-by: Alexander Potapenko <glider@google.com>
> ---
>   mm/internal.h |  4 ++--
>   mm/memblock.c | 18 +++++++++---------
>   mm/mm_init.c  |  9 +++++----
>   3 files changed, 16 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index 45b725c3dc030..ae1ee6e02eff9 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -742,8 +742,8 @@ static inline void clear_zone_contiguous(struct zone *zone)
>   extern int __isolate_free_page(struct page *page, unsigned int order);
>   extern void __putback_isolated_page(struct page *page, unsigned int order,
>   				    int mt);
> -extern void memblock_free_pages(struct page *page, unsigned long pfn,
> -					unsigned int order);
> +extern unsigned long memblock_free_pages(struct page *page, unsigned long pfn,
> +					 unsigned int order);
>   extern void __free_pages_core(struct page *page, unsigned int order,
>   		enum meminit_context context);
>   
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 117d963e677c9..de7ff644d8f4f 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -1834,10 +1834,9 @@ void __init memblock_free_late(phys_addr_t base, phys_addr_t size)
>   	cursor = PFN_UP(base);
>   	end = PFN_DOWN(base + size);
>   
> -	for (; cursor < end; cursor++) {
> -		memblock_free_pages(pfn_to_page(cursor), cursor, 0);
> -		totalram_pages_inc();
> -	}
> +	for (; cursor < end; cursor++)
> +		totalram_pages_add(
> +			memblock_free_pages(pfn_to_page(cursor), cursor, 0));
>   }

That part is clear. But for readability we should probably just do

if (memblock_free_pages(pfn_to_page(cursor), cursor, 0))
	totalram_pages_inc();

Or use a temp variable as an alternative.


LGTM

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Cheers

David / dhildenb