+ mm-sparse-vmemmap-factor-out-shared-vmemmap-tail-page-allocation.patch added to mm-new branch

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

The patch titled
     Subject: mm/sparse-vmemmap: factor out shared vmemmap tail page allocation
has been added to the -mm mm-new branch.  Its filename is
     mm-sparse-vmemmap-factor-out-shared-vmemmap-tail-page-allocation.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-factor-out-shared-vmemmap-tail-page-allocation.patch

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

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

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------------------------------------------------------
From: Muchun Song <songmuchun@bytedance.com>
Subject: mm/sparse-vmemmap: factor out shared vmemmap tail page allocation
Date: Fri, 11 Sep 2026 13:02:19 +0800

HugeTLB and sparse-vmemmap each have their own helper to allocate the
shared vmemmap tail page used by vmemmap optimization.

Factor that logic into a common vmemmap_shared_tail_page() helper.  It
allocates the page through vmemmap_alloc_block(), and uses cmpxchg() to
install the per-zone shared page.

Expose zone->vmemmap_tails under CONFIG_SPARSEMEM_VMEMMAP to match the
shared helper's build condition.  This avoids a
!CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION stub; when optimization is
disabled, the array has no entries and the compiler folds away the unused
paths, so no storage or runtime overhead is added.

This removes duplicate allocation logic while still handling both the
early boot and runtime paths through the same helper.

Link: https://lore.kernel.org/20260911050228.58884-3-songmuchun@bytedance.com
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Qi Zheng <qi.zheng@linux.dev>
Cc: David Hildenbrand (Arm) <david@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/mmzone.h |    2 -
 mm/hugetlb_vmemmap.c   |   28 --------------
 mm/sparse-vmemmap.c    |   74 +++++++++++++++------------------------
 mm/sparse.h            |    1 
 4 files changed, 33 insertions(+), 72 deletions(-)

--- a/include/linux/mmzone.h~mm-sparse-vmemmap-factor-out-shared-vmemmap-tail-page-allocation
+++ a/include/linux/mmzone.h
@@ -1156,7 +1156,7 @@ struct zone {
 	/* Zone statistics */
 	atomic_long_t		vm_stat[NR_VM_ZONE_STAT_ITEMS];
 	atomic_long_t		vm_numa_event[NR_VM_NUMA_EVENT_ITEMS];
-#ifdef CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION
+#ifdef CONFIG_SPARSEMEM_VMEMMAP
 	struct page *vmemmap_tails[VMEMMAP_OPTIMIZATION_NR_ORDERS];
 #endif
 } ____cacheline_internodealigned_in_smp;
--- a/mm/hugetlb_vmemmap.c~mm-sparse-vmemmap-factor-out-shared-vmemmap-tail-page-allocation
+++ a/mm/hugetlb_vmemmap.c
@@ -493,32 +493,6 @@ static bool vmemmap_should_optimize_foli
 	return true;
 }
 
-static struct page *vmemmap_get_tail(unsigned int order, struct zone *zone)
-{
-	const unsigned int idx = order - VMEMMAP_OPTIMIZATION_MIN_ORDER;
-	struct page *tail, *p;
-	int node = zone_to_nid(zone);
-
-	tail = READ_ONCE(zone->vmemmap_tails[idx]);
-	if (likely(tail))
-		return tail;
-
-	tail = alloc_pages_node(node, GFP_KERNEL | __GFP_ZERO, 0);
-	if (!tail)
-		return NULL;
-
-	p = page_to_virt(tail);
-	for (int i = 0; i < PAGE_SIZE / sizeof(struct page); i++)
-		init_compound_tail(p + i, NULL, order, zone);
-
-	if (cmpxchg(&zone->vmemmap_tails[idx], NULL, tail)) {
-		__free_page(tail);
-		tail = READ_ONCE(zone->vmemmap_tails[idx]);
-	}
-
-	return tail;
-}
-
 static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
 					    struct folio *folio,
 					    struct list_head *vmemmap_pages,
@@ -535,7 +509,7 @@ static int __hugetlb_vmemmap_optimize_fo
 		return ret;
 
 	nid = folio_nid(folio);
-	vmemmap_tail = vmemmap_get_tail(h->order, folio_zone(folio));
+	vmemmap_tail = vmemmap_shared_tail_page(h->order, folio_zone(folio));
 	if (!vmemmap_tail)
 		return -ENOMEM;
 
--- a/mm/sparse.h~mm-sparse-vmemmap-factor-out-shared-vmemmap-tail-page-allocation
+++ a/mm/sparse.h
@@ -142,6 +142,7 @@ static inline void sparse_sections_init(
  * mm/sparse-vmemmap.c
  */
 #ifdef CONFIG_SPARSEMEM_VMEMMAP
+struct page *vmemmap_shared_tail_page(unsigned int order, struct zone *zone);
 void sparse_init_subsection_map(void);
 int section_nr_vmemmap_pages(unsigned long pfn, unsigned long nr_pages,
 		struct vmem_altmap *altmap, struct dev_pagemap *pgmap);
--- a/mm/sparse-vmemmap.c~mm-sparse-vmemmap-factor-out-shared-vmemmap-tail-page-allocation
+++ a/mm/sparse-vmemmap.c
@@ -42,27 +42,13 @@
 #include "mm_init.h"
 #include "sparse.h"
 
-/*
- * Allocate a block of memory to be used to back the virtual memory map
- * or to back the page tables that are used to create the mapping.
- * Uses the main allocators if they are available, else bootmem.
- */
-
-static void * __ref __earlyonly_bootmem_alloc(int node,
-				unsigned long size,
-				unsigned long align,
-				unsigned long goal)
-{
-	return memmap_alloc(size, align, goal, node, false);
-}
-
-void * __meminit vmemmap_alloc_block(unsigned long size, int node)
+void __ref *vmemmap_alloc_block(unsigned long size, int node)
 {
 	/* If the main allocator is up use that, fallback to bootmem. */
 	if (slab_is_available()) {
 		gfp_t gfp_mask = GFP_KERNEL|__GFP_RETRY_MAYFAIL|__GFP_NOWARN;
 		int order = get_order(size);
-		static bool warned __meminitdata;
+		static bool warned;
 		struct page *page;
 
 		page = alloc_pages_node(node, gfp_mask, order);
@@ -76,8 +62,7 @@ void * __meminit vmemmap_alloc_block(uns
 		}
 		return NULL;
 	} else
-		return __earlyonly_bootmem_alloc(node, size, size,
-				__pa(MAX_DMA_ADDRESS));
+		return memmap_alloc(size, size, __pa(MAX_DMA_ADDRESS), node, false);
 }
 
 static void * __meminit altmap_alloc_block_buf(unsigned long size,
@@ -184,39 +169,40 @@ static void * __meminit vmemmap_alloc_bl
 	return p;
 }
 
-#ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
-static __meminit struct page *vmemmap_get_tail(unsigned int order, struct zone *zone)
+struct page __ref *vmemmap_shared_tail_page(unsigned int order, struct zone *zone)
 {
-	struct page *p, *tail;
-	unsigned int idx;
-	int node = zone_to_nid(zone);
+	void *addr;
+	struct page *page;
+	const unsigned int idx = order - VMEMMAP_OPTIMIZATION_MIN_ORDER;
 
-	if (WARN_ON_ONCE(order < VMEMMAP_OPTIMIZATION_MIN_ORDER))
-		return NULL;
-	if (WARN_ON_ONCE(order > MAX_FOLIO_ORDER))
+	if (WARN_ON_ONCE(idx >= ARRAY_SIZE(zone->vmemmap_tails)))
 		return NULL;
 
-	idx = order - VMEMMAP_OPTIMIZATION_MIN_ORDER;
-	tail = zone->vmemmap_tails[idx];
-	if (tail)
-		return tail;
-	p = vmemmap_alloc_block_zero(PAGE_SIZE, node);
-	if (!p)
+	page = READ_ONCE(zone->vmemmap_tails[idx]);
+	if (likely(page))
+		return page;
+
+	addr = vmemmap_alloc_block(PAGE_SIZE, zone_to_nid(zone));
+	if (!addr)
 		return NULL;
-	for (int i = 0; i < PAGE_SIZE / sizeof(struct page); i++)
-		init_compound_tail(p + i, NULL, order, zone);
 
-	tail = virt_to_page(p);
-	zone->vmemmap_tails[idx] = tail;
+	for (int i = 0; i < PAGE_SIZE / sizeof(struct page); i++) {
+		page = (struct page *)addr + i;
+		mm_zero_struct_page(page);
+		init_compound_tail(page, NULL, order, zone);
+	}
 
-	return tail;
-}
-#else
-static inline struct page *vmemmap_get_tail(unsigned int order, struct zone *zone)
-{
-	return NULL;
+	page = virt_to_page(addr);
+	if (cmpxchg(&zone->vmemmap_tails[idx], NULL, page) != NULL) {
+		if (slab_is_available())
+			__free_page(page);
+		else
+			memblock_free(addr, PAGE_SIZE);
+		page = READ_ONCE(zone->vmemmap_tails[idx]);
+	}
+
+	return page;
 }
-#endif
 
 static __meminit void *vmemmap_alloc_pte(unsigned long pfn, int node,
 					 struct vmem_altmap *altmap)
@@ -229,7 +215,7 @@ static __meminit void *vmemmap_alloc_pte
 		return vmemmap_alloc_block_buf(PAGE_SIZE, node, altmap);
 
 	zone = pfn_to_zone(pfn, node);
-	page = vmemmap_get_tail(order, zone);
+	page = vmemmap_shared_tail_page(order, zone);
 	if (!page)
 		return NULL;
 
_

Patches currently in -mm which might be from songmuchun@bytedance.com are

mm-sparse-relax-struct-mem_section-size-constraints.patch
mm-sparse-vmemmap-rename-hvo-order-macros.patch
mm-mm_init-skip-initializing-shared-vmemmap-tail-pages.patch
mm-sparse-vmemmap-initialize-shared-tail-vmemmap-pages-on-allocation.patch
mm-sparse-vmemmap-support-section-based-vmemmap-accounting.patch
mm-mm_init-factor-out-pfn_to_zone.patch
mm-sparse-vmemmap-move-helpers-ahead-of-future-callers.patch
mm-sparse-vmemmap-support-section-based-vmemmap-optimization.patch
mm-sparse-initialize-memory-sections-earlier.patch
mm-hugetlb-switch-hugetlb-to-section-based-vmemmap-optimization.patch
mm-sparse-vmemmap-remove-sparsemem_vmemmap_preinit-support.patch
mm-sparse-inline-usemap-allocation-into-sparse_init_nid.patch
mm-sparse-remove-section_map_size.patch
mm-hugetlb-remove-huge_bootmem_hvo.patch
mm-hugetlb-remove-huge_bootmem_cma.patch
mm-hugetlb-localize-struct-huge_bootmem_page.patch
mm-hugetlb-localize-huge_bootmem_zones_valid.patch
mm-sparse-vmemmap-introduce-config_sparsemem_vmemmap_optimization.patch
mm-sparse-vmemmap-factor-out-shared-vmemmap-tail-page-allocation.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