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

[Anshuman Khandual]

On 4/12/21 2:17 PM, David Hildenbrand wrote:
> On 12.04.21 10:06, Anshuman Khandual wrote:
>> + linuxppc-dev@xxxxxxxxxxxxxxxx
>> + linux-ia64@xxxxxxxxxxxxxxx
>>
>> 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@xxxxxxxxxx>
>>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>>> Cc: linux-mm@xxxxxxxxx
>>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
>>> ---
>>> 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@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 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")

Right.

> 
> 
> 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).

Right.

> 
> 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?

Agreed.

> 
> Having pageblock_order > MAX_ORDER feels wrong and looks shaky.
> 
Agreed, definitely does not look right. Lets see what other folks
might have to say on this.

+ Christoph Lameter <cl@xxxxxxxxx>