Re: [PATCH] mm/compaction: fix low_pfn advance on isolating hugetlb

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

On Thu, Sep 11, 2025 at 09:29:39PM -0400, Zi Yan wrote:
[...]
>>>
>>> For compound pages, we will always have tail pfn < head pfn, so we should
>>> always find the head page first.
>>>
>>
>> I think you want to say tail pfn > head pfn?
>>
>>> If you did find a case where we somehow encounter a tail page here, I'd
>>> love to see it. And then you'd also want to make sure the other compaction
>>> trackers are appropriately accounted for.
>>
>> I may not follow you here, below is the call flow for
>> isolate_migratepages_block() invoked during __alloc_contig_pages().
>>
>> __alloc_contig_pages(nr_pages, ..);
>>     start = ALIGN(zone->zone_start_pfn, nr_pages);
>>     alloc_contig_range_noprof(start, ..);
>>         __alloc_contig_migrate_range(.., start, ..);
>>             pfn = start;
>>             isolate_migratepages_range(.., pfn, ..);
>>                 isolate_migratepages_block(.., pfn, ..);
>>                     page = pfn_to_page(pfn);
>>     start += nr_pages;
>>
>> In the loop of __alloc_contig_pages(), it iterate on each nr_pages range. And
>> nr_pages seems could be any positive number, so it looks the first pfn checked
>> by isolate_migratepages_block() could be not aligned with page order or less
>> than MAX_PAGE_ORDER. This mean it could be a tail page per my understanding.
>>
>> Maybe I missed some point here?
>
>You are right.
>
>But nr_pages cannot be any positive number, since ALIGN only accepts
>power of 2 as the alignment. So alloc_contig_pages() might need another
>fix to handle the case nr_pages is not power of 2.
>

You are right, I missed ALIGN() requirement.

So here we should use roundup().

>Oh, after I checked pfn_range_valid_contig(), I find your example does not
>apply, since it returns false when any page in the range is PageHuge.

You are right, here hide some magic...

>This means with your example, PageHuge branch will never be executed.
>But alloc_contig_range_noprof() is exported and can be used directly,
>the @start input can be any pfn, which can be in the middle of PageHuge.
>So your fix is still needed for this case.
>

Then the next question is why we filter PageHuge() in
pfn_range_valid_contig(). Looks we can handle PageHuge() by isolating and
migrate it.

>--
>Best Regards,
>Yan, Zi

-- 
Wei Yang
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