Re: [PATCH v2] mm/memblock: Correct totalram_pages accounting with KMSAN

[SeongJae Park]

Hello,

On Wed, 24 Sep 2025 12:03:01 +0200 Alexander Potapenko <glider@xxxxxxxxxx> 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@xxxxxxxxxx>
> Fixes: 3c2065098260 ("init: kmsan: call KMSAN initialization routines")
> Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
[...]
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -2548,24 +2548,25 @@ void *__init alloc_large_system_hash(const char *tablename,
>  	return table;
>  }
>  
> -void __init memblock_free_pages(struct page *page, unsigned long pfn,
> -							unsigned int order)
> +unsigned long __init memblock_free_pages(struct page *page, unsigned long pfn,
> +					 unsigned int order)
>  {
>  	if (IS_ENABLED(CONFIG_DEFERRED_STRUCT_PAGE_INIT)) {
>  		int nid = early_pfn_to_nid(pfn);
>  
>  		if (!early_page_initialised(pfn, nid))
> -			return;
> +			return 0;
>  	}

I found this patch on mm-new tree is making my test machine (QEMU) reports much
less MemTotal even though KMSAN is disabled.  And modifying the above part to
be considered as free success (returning '1UL << order') fixed my issue.
Because the commit message says the purpose of this change is only for
KMSAN-stashed memory, maybe the above behavior change is not really intended?

I'm not familiar with this code so I'm unsure if the workaround is the right
fix.  But since I have no time to look this in deep for now, reporting first.

>  
>  	if (!kmsan_memblock_free_pages(page, order)) {
>  		/* KMSAN will take care of these pages. */
> -		return;
> +		return 0;
>  	}

I understand this part is the intended change, of course.


Thanks,
SJ

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