Re: [PATCH v3 3/5] mm/memory_hotplug: use pfn math in place of direct struct page manipulation.

[Zi Yan]

On 13 Sep 2023, at 16:12, Zi Yan wrote:

> From: Zi Yan <ziy@xxxxxxxxxx>
>
> When dealing with hugetlb pages, manipulating struct page pointers
> directly can get to wrong struct page, since struct page is not guaranteed
> to be contiguous on SPARSEMEM without VMEMMAP. Use pfn calculation to
> handle it properly.
>
> Fixes: eeb0efd071d8 ("mm,memory_hotplug: fix scan_movable_pages() for gigantic hugepages")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> Reviewed-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  mm/memory_hotplug.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 1b03f4ec6fd2..3b301c4023ff 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1689,7 +1689,7 @@ static int scan_movable_pages(unsigned long start, unsigned long end,
>  		 */
>  		if (HPageMigratable(head))
>  			goto found;
> -		skip = compound_nr(head) - (page - head);
> +		skip = compound_nr(head) - (pfn - page_to_pfn(head));
>  		pfn += skip - 1;
>  	}
>  	return -ENOENT;
> -- 
> 2.40.1

Without the fix, a wrong number of page might be skipped. Since skip cannot be
negative, scan_movable_page() will end early and might miss a movable page with
-ENOENT. This might fail offline_pages(). No bug is reported. The fix comes
from code inspection.

--
Best Regards,
Yan, Zi

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