linux-kernel - Re: [PATCH v5] page_alloc: allow migration of smaller hugepages during contig

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Message-ID: <aURnLqMziaOilLCu@gourry-fedora-PF4VCD3F>
Date: Thu, 18 Dec 2025 15:42:22 -0500
From: Gregory Price <gourry@...rry.net>
To: Zi Yan <ziy@...dia.com>
Cc: 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,
	richard.weiyang@...il.com, osalvador@...e.de, rientjes@...gle.com,
	david@...hat.com, joshua.hahnjy@...il.com, fvdl@...gle.com
Subject: Re: [PATCH v5] page_alloc: allow migration of smaller hugepages
 during contig_alloc

On Thu, Dec 18, 2025 at 02:45:37PM -0500, Zi Yan wrote:
> 
> That can save another scan? And caller can pass hugetlb_search_result if
> they care and check its value if pfn_range_valid_contig() returns false.
> 

Well, first, I've generally seen it discouraged to do output-parameters
like this for such trivial things.  But that aside...

We have to scan again either way if we want to prefer allocating
non-hugetlb regions in different memory blocks first.  This is what Mel
was pointing out (we should touch every OTHER block before we attempt
HugeTLB migrations).

The best optimization you could hope for is something like the following
- but honestly, this is ugly, racy (zone contents may have changed
between scans), and if you're already in the slow reliable path then we
should just be slow and re-scan the non-hugetlb sections as well.

Other than this being ugly, I don't have strong feelings.  If people
would prefer the second pass to ONLY touch hugetlb sections, I'll ship
this.

static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
                                   unsigned long nr_pages, bool search_hugetlb,
                                   bool *hugetlb_found)
{
        bool hugetlb = false;

        for (i = start_pfn; i < end_pfn; i++) {
	...
                if (PageHuge(page)) {
                        if (hugetlb_found)
                                *hugetlb_found = true;

                        if (!search_hugetlb)
                                return false;

			...
                        hugetlb = true;
                }
        }
	/* 
	 * If we're searching for hugetlb regions, only return those
	 * Otherwise only return regions without hugetlb reservations
	 */
        return !search_hugetlb || hugetlb;
}


struct page *alloc_contig_pages_noprof(unsigned long nr_pages, gfp_t gfp_mask,
                                 int nid, nodemask_t *nodemask)
{
        bool search_hugetlb = false;
	bool hugetlb_found = false;

retry:
        zonelist = node_zonelist(nid, gfp_mask);
        for_each_zone_zonelist_nodemask(zone, z, zonelist,
                                        gfp_zone(gfp_mask), nodemask) {
                spin_lock_irqsave(&zone->lock, flags);

                pfn = ALIGN(zone->zone_start_pfn, nr_pages);
                while (zone_spans_last_pfn(zone, pfn, nr_pages)) {
                        if (pfn_range_valid_contig(zone, pfn, nr_pages,
                                                   search_hugetlb,
                                                   &hugetlb_found)) {
						   ...
                }
        }
        if (IS_ENABLED(CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION) &&
            !search_hugetlb && hugetlb_found) {
                search_hugetlb = true;
                goto retry;
        }
        return NULL;
}

~Gregory