Re: [PATCH V2] mm/page_alloc: Ensure that HUGETLB_PAGE_ORDER is less than MAX_ORDER

[David Hildenbrand <david@redhat.com>]

On 12.04.21 10:06, Anshuman Khandual wrote:
> + linuxppc-dev@lists.ozlabs.org
> + linux-ia64@vger.kernel.org
> 
> On 4/12/21 9:18 AM, 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: David Hildenbrand <david@redhat.com>
>> 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>
>> ---
>> Changes in V2:
>>
>> - Changed WARN_ON() to BUILD_BUG_ON() per David
>>
>> Changes in V1:
>>
>> https://patchwork.kernel.org/project/linux-mm/patch/1617947717-2424-1-git-send-email-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 cfc72873961d..19283bff4bec 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -6875,10 +6875,17 @@ 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.
>> +		 */
>> +		BUILD_BUG_ON(HUGETLB_PAGE_ORDER >= MAX_ORDER);
> 
> Unfortunately the build test fails on both the platforms (powerpc and ia64)
> which subscribe HUGETLB_PAGE_SIZE_VARIABLE and where this check would make
> sense. I some how overlooked the cross compile build failure that actually
> detected this problem.
> 
> But wondering why this assert is not holding true ? and how these platforms
> do not see the warning during boot (or do they ?) at mm/vmscan.c:1092 like
> arm64 did.
> 
> static int __fragmentation_index(unsigned int order, struct contig_page_info *info)
> {
>          unsigned long requested = 1UL << order;
> 
>          if (WARN_ON_ONCE(order >= MAX_ORDER))
>                  return 0;
> ....
> 
> Can pageblock_order really exceed MAX_ORDER - 1 ?

Ehm, for now I was under the impression that such configurations 
wouldn't exist.

And originally, HUGETLB_PAGE_SIZE_VARIABLE was introduced to handle 
hugepage sizes that all *smaller* than MAX_ORDER - 1: See d9c234005227 
("Do not depend on MAX_ORDER when grouping pages by mobility")


However, looking into init_cma_reserved_pageblock():

	if (pageblock_order >= MAX_ORDER) {
		i = pageblock_nr_pages;
		...
	}


But it's kind of weird, isn't it? Let's assume we have MAX_ORDER - 1 
correspond to 4 MiB and pageblock_order correspond to 8 MiB.

Sure, we'd be grouping pages in 8 MiB chunks, however, we cannot even 
allocate 8 MiB chunks via the buddy. So only alloc_contig_range() could 
really grab them (IOW: gigantic pages).

Further, we have code like deferred_free_range(), where we end up 
calling __free_pages_core()->...->__free_one_page() with 
pageblock_order. Wouldn't we end up setting the buddy order to something 
 > MAX_ORDER -1 on that path?

Having pageblock_order > MAX_ORDER feels wrong and looks shaky.

-- 
Thanks,

David / dhildenb