+ mm-sparse-vmemmap-allocate-shared-tail-page-array-dynamically.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/sparse-vmemmap: allocate shared tail page array dynamically
has been added to the -mm mm-unstable branch.  Its filename is
     mm-sparse-vmemmap-allocate-shared-tail-page-array-dynamically.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-sparse-vmemmap-allocate-shared-tail-page-array-dynamically.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Muchun Song <muchun.song@linux.dev>
Subject: mm/sparse-vmemmap: allocate shared tail page array dynamically
Date: Sun, 27 Sep 2026 10:54:31 +0800

Commit 622026e87c40 ("mm/hugetlb: remove fake head pages") added the
per-zone vmemmap_tails array. Its size depends on MAX_FOLIO_ORDER, which
had been moved to mmzone.h in preparation for the array.

PUD_ORDER is defined by linux/pgtable.h, which cannot be included from
mmzone.h without creating an include cycle. It was therefore open-coded
as PUD_SHIFT - PAGE_SHIFT.

This removed the dependency on PUD_ORDER, but not the underlying
dependency on architecture page-table definitions. PUD_SHIFT is
generally provided by architecture page-table headers, which are not
guaranteed to have been included when mmzone.h is parsed.

The dependency remained hidden because vmemmap_tails was originally
guarded by CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP. Under that condition,
MAX_FOLIO_ORDER resolves to either MAX_PAGE_ORDER or the fixed HugeTLB
limit, rather than the PUD_SHIFT-based definition.

Device DAX, however, does not require CONFIG_HUGETLB_PAGE. When it is
converted to use section-based vmemmap optimization, MAX_FOLIO_ORDER can
resolve to PUD_SHIFT - PAGE_SHIFT while it is being used to size
vmemmap_tails. This would make struct zone depend on architecture
page-table definitions being available when mmzone.h is parsed.

Replace the embedded array with a pointer and allocate it on first use.
This moves the order-count evaluation into sparse-vmemmap.c, after the
architecture page-table definitions are available, and removes the
dependency from mmzone.h.

Removing the compile-time array also removes the original reason for
keeping MAX_FOLIO_ORDER and the vmemmap optimization sizing definitions
in mmzone.h. Follow-up cleanups can place each definition in the header
owned by its respective subsystem.

Link: https://lore.kernel.org/20260927025441.741633-3-songmuchun@bytedance.com
Signed-off-by: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Qi Zheng <qi.zheng@linux.dev>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Christophe Leroy <chleroy@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Lance Yang <lance.yang@linux.dev>
---

 include/linux/mmzone.h |    7 +------
 mm/sparse-vmemmap.c    |   35 +++++++++++++++++++++++++++++++----
 2 files changed, 32 insertions(+), 10 deletions(-)

--- a/include/linux/mmzone.h~mm-sparse-vmemmap-allocate-shared-tail-page-array-dynamically
+++ a/include/linux/mmzone.h
@@ -113,11 +113,6 @@
 	(VMEMMAP_OPTIMIZATION_PAGES * PAGE_SIZE / sizeof(struct page))
 #define VMEMMAP_OPTIMIZATION_MIN_ORDER		(ilog2(VMEMMAP_OPTIMIZATION_NR_STRUCT_PAGES) + 1)
 
-#define __VMEMMAP_OPTIMIZATION_NR_ORDERS	\
-	(MAX_FOLIO_ORDER - VMEMMAP_OPTIMIZATION_MIN_ORDER + 1)
-#define VMEMMAP_OPTIMIZATION_NR_ORDERS		\
-	(__VMEMMAP_OPTIMIZATION_NR_ORDERS > 0 ? __VMEMMAP_OPTIMIZATION_NR_ORDERS : 0)
-
 enum migratetype {
 	MIGRATE_UNMOVABLE,
 	MIGRATE_MOVABLE,
@@ -1156,7 +1151,7 @@ struct zone {
 	atomic_long_t		vm_stat[NR_VM_ZONE_STAT_ITEMS];
 	atomic_long_t		vm_numa_event[NR_VM_NUMA_EVENT_ITEMS];
 #ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
-	struct page *vmemmap_tails[VMEMMAP_OPTIMIZATION_NR_ORDERS];
+	struct page **vmemmap_tails;
 #endif
 } ____cacheline_internodealigned_in_smp;
 
--- a/mm/sparse-vmemmap.c~mm-sparse-vmemmap-allocate-shared-tail-page-array-dynamically
+++ a/mm/sparse-vmemmap.c
@@ -170,16 +170,43 @@ static void * __meminit vmemmap_alloc_bl
 }
 
 #ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
+#define VMEMMAP_OPTIMIZATION_NR_ORDERS	(MAX_FOLIO_ORDER - VMEMMAP_OPTIMIZATION_MIN_ORDER + 1)
+
+static __ref struct page **vmemmap_tails_alloc(struct zone *zone)
+{
+	struct page **pages;
+	const size_t size = array_size(VMEMMAP_OPTIMIZATION_NR_ORDERS, sizeof(*pages));
+
+	pages = slab_is_available() ? kzalloc_objs(*pages, VMEMMAP_OPTIMIZATION_NR_ORDERS) :
+		memblock_alloc(size, __alignof__(*pages));
+	if (!pages)
+		return NULL;
+
+	if (cmpxchg(&zone->vmemmap_tails, NULL, pages) != NULL) {
+		if (slab_is_available())
+			kfree(pages);
+		else
+			memblock_free(pages, size);
+		pages = READ_ONCE(zone->vmemmap_tails);
+	}
+
+	return pages;
+}
+
 struct page __ref *vmemmap_shared_tail_page(unsigned int order, struct zone *zone)
 {
 	void *addr;
-	struct page *page;
+	struct page *page, **pages;
 	const unsigned int idx = order - VMEMMAP_OPTIMIZATION_MIN_ORDER;
 
 	if (WARN_ON_ONCE(idx >= VMEMMAP_OPTIMIZATION_NR_ORDERS))
 		return NULL;
 
-	page = READ_ONCE(zone->vmemmap_tails[idx]);
+	pages = READ_ONCE(zone->vmemmap_tails) ? : vmemmap_tails_alloc(zone);
+	if (!pages)
+		return NULL;
+
+	page = READ_ONCE(pages[idx]);
 	if (likely(page))
 		return page;
 
@@ -194,12 +221,12 @@ struct page __ref *vmemmap_shared_tail_p
 	}
 
 	page = virt_to_page(addr);
-	if (cmpxchg(&zone->vmemmap_tails[idx], NULL, page) != NULL) {
+	if (cmpxchg(&pages[idx], NULL, page) != NULL) {
 		if (slab_is_available())
 			__free_page(page);
 		else
 			memblock_free(addr, PAGE_SIZE);
-		page = READ_ONCE(zone->vmemmap_tails[idx]);
+		page = READ_ONCE(pages[idx]);
 	}
 
 	return page;
_

Patches currently in -mm which might be from muchun.song@linux.dev are

mm-sparse-vmemmap-factor-out-shared-vmemmap-tail-page-allocation.patch
mm-sparse-vmemmap-allocate-shared-tail-page-array-dynamically.patch
mm-sparse-vmemmap-introduce-config_vmemmap_optimization.patch
mm-sparse-vmemmap-open-code-init_compound_tail.patch
mm-sparse-vmemmap-prepare-dax-vmemmap-population-for-compound-page-orders.patch
mm-sparse-vmemmap-set-compound-page-order-for-device-dax.patch
mm-sparse-vmemmap-switch-device-dax-to-shared-tail-vmemmap-pages.patch
mm-sparse-vmemmap-move-vmemmap-optimization-helpers-to-a-public-header.patch
powerpc-mm-switch-device-dax-to-shared-tail-vmemmap-pages.patch
mm-sparse-vmemmap-drop-the-extra-tail-page-from-device-dax-reservation.patch
mm-sparse-vmemmap-drop-unused-section_nr_vmemmap_pages-arguments.patch
documentation-mm-update-dax-vmemmap-deduplication-docs.patch