Message-ID: <20251218190832.1319797-1-gourry@gourry.net>
Date: Thu, 18 Dec 2025 14:08:31 -0500
From: Gregory Price <gourry@...rry.net>
To: linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org,
	kernel-team@...a.com,
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Subject: [PATCH v5] page_alloc: allow migration of smaller hugepages during contig_alloc

We presently skip regions with hugepages entirely when trying to do
contiguous page allocation.  This will cause otherwise-movable
2MB HugeTLB pages to be considered unmovable, and will make 1GB
hugepages allocation less reliable on systems utilizing both.

Commit 4d73ba5fa710 ("mm: page_alloc: skip regions with hugetlbfs pages
when allocating 1G pages") skipped all HugePage containing regions
because it can cause significant delays in 1G allocation (as HugeTLB
migrations may fail for a number of reasons).

Instead, if hugepage migration is enabled, consider regions with
hugepages smaller than the target contiguous allocation request
as valid targets for allocation.

We optimize for the existing behavior by searching for non-hugetlb
regions in a first pass, then retrying the search to include hugetlb
only on failure.  This allows the existing fast-path to remain the
default case with a slow-path fallback to increase reliability.

isolate_migrate_pages_block() has similar hugetlb filter logic, and
the hugetlb code does a migratable check in folio_isolate_hugetlb()
during isolation.  The code servicing the allocation and migration
already supports this exact use case (it's just unreachable).

To test, allocate a bunch of 2MB HugeTLB pages (in this case 48GB)
and then attempt to allocate some 1G HugeTLB pages (in this case 4GB)
(Scale to your machine's memory capacity).

echo 24576 > .../hugepages-2048kB/nr_hugepages
echo 4 > .../hugepages-1048576kB/nr_hugepages

Prior to this patch, the 1GB page allocation can fail if no contiguous
1GB pages remain.  After this patch, the kernel will try to move 2MB
pages and successfully allocate the 1GB pages (assuming overall
sufficient memory is available). Also tested this while a program had
the 2MB reservations mapped, and the 1GB reservation still succeeds.

folio_alloc_gigantic() is the primary user of alloc_contig_pages(),
other users are debug or init-time allocations and largely unaffected.
- ppc/memtrace is a debugfs interface
- x86/tdx memory allocation occurs once on module-init
- kfence/core happens once on module (late) init
- THP uses it in debug_vm_pgtable_alloc_huge_page at __init time

Suggested-by: David Hildenbrand <david@...hat.com>
Link: https://lore.kernel.org/linux-mm/6fe3562d-49b2-4975-aa86-e139c535ad00@redhat.com/
Signed-off-by: Gregory Price <gourry@...rry.net>
---
v5: add fast-path/slow-path mechanism to retain current performance
    dropped tags as this changes the behavior of the patch
    most of the logic otherwise remains the same.

 mm/page_alloc.c | 44 ++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 40 insertions(+), 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 822e05f1a964..3ddad1fca924 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7083,7 +7083,7 @@ static int __alloc_contig_pages(unsigned long start_pfn,
 }
 
 static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
-				   unsigned long nr_pages)
+				   unsigned long nr_pages, bool search_hugetlb)
 {
 	unsigned long i, end_pfn = start_pfn + nr_pages;
 	struct page *page;
@@ -7099,8 +7099,30 @@ static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
 		if (PageReserved(page))
 			return false;
 
-		if (PageHuge(page))
-			return false;
+		/*
+		 * Only consider ranges containing hugepages if those pages are
+		 * smaller than the requested contiguous region.  e.g.:
+		 *     Move 2MB pages to free up a 1GB range.
+		 *     Don't move 1GB pages to free up a 2MB range.
+		 *
+		 * This makes contiguous allocation more reliable if multiple
+		 * hugepage sizes are used without causing needless movement.
+		 */
+		if (PageHuge(page)) {
+			unsigned int order;
+
+			if (!IS_ENABLED(CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION))
+				return false;
+
+			if (!search_hugetlb)
+				return false;
+
+			page = compound_head(page);
+			order = compound_order(page);
+			if ((order >= MAX_FOLIO_ORDER) ||
+			    (nr_pages <= (1 << order)))
+				return false;
+		}
 	}
 	return true;
 }
@@ -7143,7 +7165,9 @@ struct page *alloc_contig_pages_noprof(unsigned long nr_pages, gfp_t gfp_mask,
 	struct zonelist *zonelist;
 	struct zone *zone;
 	struct zoneref *z;
+	bool hugetlb = false;
 
+retry:
 	zonelist = node_zonelist(nid, gfp_mask);
 	for_each_zone_zonelist_nodemask(zone, z, zonelist,
 					gfp_zone(gfp_mask), nodemask) {
@@ -7151,7 +7175,8 @@ struct page *alloc_contig_pages_noprof(unsigned long nr_pages, gfp_t gfp_mask,
 
 		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)) {
+			if (pfn_range_valid_contig(zone, pfn, nr_pages,
+						   hugetlb)) {
 				/*
 				 * We release the zone lock here because
 				 * alloc_contig_range() will also lock the zone
@@ -7170,6 +7195,17 @@ struct page *alloc_contig_pages_noprof(unsigned long nr_pages, gfp_t gfp_mask,
 		}
 		spin_unlock_irqrestore(&zone->lock, flags);
 	}
+	/*
+	 * If we failed, retry the search, but treat regions with HugeTLB pages
+	 * as valid targets.  This retains fast-allocations on first pass
+	 * without trying to migrate HugeTLB pages (which may fail). On the
+	 * second pass, we will try moving HugeTLB pages when those pages are
+	 * smaller than the requested contiguous region size.
+	 */
+	if (!hugetlb && IS_ENABLED(CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION)) {
+		hugetlb = true;
+		goto retry;
+	}
 	return NULL;
 }
 #endif /* CONFIG_CONTIG_ALLOC */
-- 
2.52.0

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