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Subject: Re: [PATCH] mm/page_alloc: Ensure that HUGETLB_PAGE_ORDER is less
 than MAX_ORDER
To:     Anshuman Khandual <anshuman.khandual@arm.com>, linux-mm@kvack.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org
References: <1617947717-2424-1-git-send-email-anshuman.khandual@arm.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <5a92f302-1917-1374-a8c7-0badb9ec1b8c@redhat.com>
Date:   Fri, 9 Apr 2021 10:24:18 +0200
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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@linux-foundation.org>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
>   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