Re: [PATCH] mm/page_alloc: Ensure that HUGETLB_PAGE_ORDER is less than MAX_ORDER

[David Hildenbrand]

On 09.04.21 07:55, Anshuman Khandual wrote:
> pageblock_order must always be less than MAX_ORDER, otherwise it might lead
> to an warning during boot. A similar problem got fixed on arm64 platform
> with the commit 79cc2ed5a716 ("arm64/mm: Drop THP conditionality from
> FORCE_MAX_ZONEORDER"). Assert the above condition before HUGETLB_PAGE_ORDER
> gets assigned as pageblock_order. This will help detect the problem earlier
> on platforms where HUGETLB_PAGE_SIZE_VARIABLE is enabled.
>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> ---
>   mm/page_alloc.c | 11 +++++++++--
>   1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 604dcd69397b..81b7460e1228 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7068,10 +7068,17 @@ void __init set_pageblock_order(void)
>   	if (pageblock_order)
>   		return;
>   
> -	if (HPAGE_SHIFT > PAGE_SHIFT)
> +	if (HPAGE_SHIFT > PAGE_SHIFT) {
> +		/*
> +		 * pageblock_order must always be less than
> +		 * MAX_ORDER. So does HUGETLB_PAGE_ORDER if
> +		 * that is being assigned here.
> +		 */
> +		WARN_ON(HUGETLB_PAGE_ORDER >= MAX_ORDER);

Can't that be a BUILD_BUG_ON() ?

>   		order = HUGETLB_PAGE_ORDER;
> -	else
> +	} else {
>   		order = MAX_ORDER - 1;
> +	}
>   
>   	/*
>   	 * Assume the largest contiguous order of interest is a huge page.


--
Thanks,

David / dhildenb