Re: [PATCH] mm/page_alloc: check the correct buddy if it is a starting block

[Wei Yang <richard.weiyang@gmail.com>]

On Thu, Sep 04, 2025 at 10:01:01PM -0400, Zi Yan wrote:
>On 3 Sep 2025, at 22:06, Wei Yang wrote:
>
>> find_large_buddy() search buddy based on start_pfn, which maybe
>> different from page's pfn, e.g. when page is not pageblock aligned,
>> because prep_move_freepages_block() always align start_pfn to pageblock.
>>
>> This means when we found a starting block at start_pfn, it may check
>> on the wrong page theoretically.
>
>and not split the free page as it is supposed to, causing a freelist migratetype
>mismatch.
>

Thanks, this is important.

>>
>> The good news is the page passed to __move_freepages_block_isolate() has
>> only two possible cases:
>>
>>   * page is pageblock aligned
>>   * page is __first_valid_page() of this block
>>
>> So it is safe for the first case, and it won't get a buddy larger than
>> pageblock for the second case.
>>
>> To eliminate the ambiguity, unify the handling for starting/tail block.
>
>To fix the issue, check the returned pfn of find_large_buddy() to decide
>whether to split the free page:
>1. if it is not a PageBuddy pfn, no split;
>2. if it is a PageBuddy pfn but order <= pageblock_order, no split;
>3. if it is a PageBuddy pfn with order > pageblock_order,
>   start_pfn is either in the starting block or tail block, split the PageBuddy
>   at pageblock_order level.
>
>
>Otherwise, LGTM. Reviewed-by: Zi Yan <ziy@nvidia.com>

Thanks.

@Andrew, if not bother, would you mind adjust the changelog?

-- 
Wei Yang
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