Re: [PATCH v6] page_alloc: allow migration of smaller hugepages during contig_alloc

[Zi Yan]

On 19 Dec 2025, at 9:26, Gregory Price wrote:

> On Fri, Dec 19, 2025 at 12:08:00AM +0000, Wei Yang wrote:
>>> +
>>> +			page = compound_head(page);
>>> +			order = compound_order(page);
>>
>> The order is get from head page.
>>
>>> +			if ((order >= MAX_FOLIO_ORDER) ||
>>> +			    (nr_pages <= (1 << order)))
>>> +				return false;
>>> +
>>> +			/* No need to check the pfns for this page */
>>> +			i += (1 << order) - 1;
>>
>> So this advance should based on "head page" instead of original page, right?
>>
>
> hm, I think the thought here was that since we're moving forward from
> start of an aligned chunk, we'd never hit a non-head page - but this
> may not be true.
>
> Will think about this for a bit.

The sole caller of pfn_range_valid_contig(), alloc_contig_pages_noprof(),
scans from the beginning of a zone to the end. pfn_range_valid_contig()
should see head pages all the time, except it scans in the middle of
a 1GB hugetlb when alloc_contig_pages_noprof() is asking for a smaller
nr_pages, like 2MB. But in that case, the if above i += (1 << order) - 1
would return false without reaching it. Basically, to get to
i += ..., pfn_range_valid_contig() needs to search for nr_pages larger
than PageHuge(page) and nr_pages is always power of two based on
alloc_contig_pages_noprof() requirement, but that means
pfn_range_valid_contig() always sees such PageHuge pages as a whole
within nr_pages range, thus cannot see a tail PageHuge page at the
point of i += ....

Best Regards,
Yan, Zi