linux-kernel - Re: [RFC PATCH] mm, hugetlb: implement movable_gigantic

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Message-ID: <aPZkzY6cGfdKo8K8@gourry-fedora-PF4VCD3F>
Date: Mon, 20 Oct 2025 12:35:25 -0400
From: Gregory Price <gourry@...rry.net>
To: David Hildenbrand <david@...hat.com>
Cc: linux-mm@...ck.org, osalvador@...e.de, corbet@....net,
	muchun.song@...ux.dev, akpm@...ux-foundation.org,
	hannes@...xchg.org, laoar.shao@...il.com, mclapinski@...gle.com,
	joel.granados@...nel.org, jack@...e.cz, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, Mel Gorman <mgorman@...e.de>,
	Michal Hocko <mhocko@...e.com>,
	Alexandru Moise <00moses.alexander00@...il.com>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [RFC PATCH] mm, hugetlb: implement movable_gigantic_pages sysctl

On Mon, Oct 20, 2025 at 12:05:41PM -0400, Gregory Price wrote:
> On Mon, Oct 20, 2025 at 04:17:06PM +0200, David Hildenbrand wrote:
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 5549b32cdd31..5def2c53092e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6922,8 +6922,12 @@ static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
>                 if (PageReserved(page))
>                         return false;
> 
> -               if (PageHuge(page))
> -                       return false;
> +               if (PageHuge(page)) {
> +                       /* Don't consider moving same size/larger pages */
> +                       if (!CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION ||
> +                           ((1 << compound_order(page)) >= nr_pages))
> +                               return false;
> +               }
>         }
>         return true;
>  }

Quick spot-check of the compaction code suggests this is handled
essentially the same way - and in fact compaction expects
alloc_contig to be sending compaction requests w/ hugepages.

So I'll go ahead and submit this separately for discussion.

~Gregory

---

->isolate_migratepages_range()
->isolate_migratepages_block()
{
...
	if (PageHuge(page)) {
		const unsigned int order = compound_order(page);
		/*
		 * skip hugetlbfs if we are not compacting for pages
		 * bigger than its order. THPs and other compound pages
		 * are handled below.
		 */
		 ...
		/* for alloc_contig case */
		if (locked) {
			unlock_page_lruvec_irqrestore(locked, flags);
			locked = NULL;
		}

	 }
...
	/*
	 * Regardless of being on LRU, compound pages such as THP
	 * (hugetlbfs is handled above) are not to be compacted unless
	 * we are attempting an allocation larger than the compound
	 * page size. We can potentially save a lot of iterations if we
	 * skip them at once. The check is racy, but we can consider
	 * only valid values and the only danger is skipping too much.
	 */
	if (PageCompound(page) && !cc->alloc_contig) {
		const unsigned int order = compound_order(page);

		/* Skip based on page order and compaction target order. */
		if (skip_isolation_on_order(order, cc->order)) {
			if (order <= MAX_PAGE_ORDER) {
				low_pfn += (1UL << order) - 1;
				nr_scanned += (1UL << order) - 1;
			}
			goto isolate_fail;
		}
	}
}