linux-kernel - Re: [PATCH v6] page_alloc: allow migration of smaller hugepages during contig

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Message-ID: <7EED2D83-AE17-49CB-BDB6-954793EAFDBF@nvidia.com>
Date: Fri, 19 Dec 2025 15:46:25 -0500
From: Zi Yan <ziy@...dia.com>
To: Gregory Price <gourry@...rry.net>
Cc: Wei Yang <richard.weiyang@...il.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, kernel-team@...a.com,
 akpm@...ux-foundation.org, vbabka@...e.cz, surenb@...gle.com,
 mhocko@...e.com, jackmanb@...gle.com, hannes@...xchg.org, osalvador@...e.de,
 rientjes@...gle.com, david@...hat.com, joshua.hahnjy@...il.com,
 fvdl@...gle.com
Subject: Re: [PATCH v6] page_alloc: allow migration of smaller hugepages
 during contig_alloc

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