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

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Message-ID: <01A47A14-3B5F-408B-AC37-64E36AFCF14C@nvidia.com>
Date: Thu, 18 Dec 2025 16:17:14 -0500
From: Zi Yan <ziy@...dia.com>
To: Gregory Price <gourry@...rry.net>
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 18 Dec 2025, at 15:42, Gregory Price wrote:

> 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).

OK, you assume hugetlb is harder to migrate compared to other movable pages.
Considering the limited number of hugetlb pages, it is quite possible.
Anyway, I will wait for your v6. Thank you for the explanation and the
prototype below.

>
> 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


Best Regards,
Yan, Zi