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Subject: Re: [PATCH] mm/page_alloc: Ensure that HUGETLB_PAGE_ORDER is less
 than MAX_ORDER
To:     David Hildenbrand <david@redhat.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>
 <5a92f302-1917-1374-a8c7-0badb9ec1b8c@redhat.com>
From:   Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <2fde38dd-7556-4651-8623-321066e48861@arm.com>
Date:   Fri, 9 Apr 2021 14:07:59 +0530
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On 4/9/21 1:54 PM, David Hildenbrand wrote:
> 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() ?

Yes, it can be. Probably might be appropriate as well, given that both
the arguments here are compile time constants. Okay, will change.

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