+ mm-sparse-vmemmap-drop-the-extra-tail-page-from-device-dax-reservation.patch added to mm-new branch

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

The patch titled
     Subject: mm/sparse-vmemmap: drop the extra tail page from device DAX reservation
has been added to the -mm mm-new branch.  Its filename is
     mm-sparse-vmemmap-drop-the-extra-tail-page-from-device-dax-reservation.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-drop-the-extra-tail-page-from-device-dax-reservation.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: drop the extra tail page from device DAX reservation
Date: Tue, 8 Sep 2026 11:03:33 +0800

The device DAX vmemmap population still reserves one extra tail vmemmap
page after the head page.

Drop that extra reservation and let the shared tail page cover all tail
vmemmap pages after the head page, so DAX follows the same reservation
model as HugeTLB.

This reduces the reserved vmemmap pages for optimized DAX mappings to one
and removes the now-unneeded first-tail population from the generic and
powerpc paths to simplify the code as well.

Link: https://lore.kernel.org/20260908030335.96549-10-songmuchun@bytedance.com
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Qi Zheng <qi.zheng@linux.dev>
Cc: David Hildenbrand <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>
---

 arch/powerpc/mm/book3s64/radix_pgtable.c |   46 ---------------------
 include/linux/mm.h                       |    3 -
 mm/mm_init.c                             |    2 
 mm/sparse-vmemmap.c                      |   13 +----
 4 files changed, 7 insertions(+), 57 deletions(-)

--- a/arch/powerpc/mm/book3s64/radix_pgtable.c~mm-sparse-vmemmap-drop-the-extra-tail-page-from-device-dax-reservation
+++ a/arch/powerpc/mm/book3s64/radix_pgtable.c
@@ -1218,39 +1218,6 @@ base_mapping:
 	return 0;
 }
 
-static pte_t * __meminit radix__vmemmap_populate_address(unsigned long addr, int node,
-							 struct vmem_altmap *altmap,
-							 struct page *reuse)
-{
-	pgd_t *pgd;
-	p4d_t *p4d;
-	pud_t *pud;
-	pmd_t *pmd;
-	pte_t *pte;
-
-	pgd = pgd_offset_k(addr);
-	p4d = p4d_offset(pgd, addr);
-	pud = vmemmap_pud_alloc(p4d, node, addr);
-	if (!pud)
-		return NULL;
-	pmd = vmemmap_pmd_alloc(pud, node, addr);
-	if (!pmd)
-		return NULL;
-	if (pmd_leaf(*pmd))
-		/*
-		 * The second page is mapped as a hugepage due to a nearby request.
-		 * Force our mapping to page size without deduplication
-		 */
-		return NULL;
-	pte = vmemmap_pte_alloc(pmd, node, addr);
-	if (!pte)
-		return NULL;
-	radix__vmemmap_pte_populate(pmd, addr, node, NULL, NULL);
-	vmemmap_verify(pte, node, addr, addr + PAGE_SIZE);
-
-	return pte;
-}
-
 int __meminit vmemmap_populate_compound_pages(unsigned long start_pfn,
 					      unsigned long start,
 					      unsigned long end, int node,
@@ -1297,7 +1264,7 @@ int __meminit vmemmap_populate_compound_
 		if (!pte_none(*pte)) {
 			/*
 			 * This could be because we already have a compound
-			 * page whose VMEMMAP_RESERVE_NR pages were mapped and
+			 * page whose retained vmemmap page was mapped and
 			 * this request fall in those pages.
 			 */
 			next = addr + PAGE_SIZE;
@@ -1318,16 +1285,7 @@ int __meminit vmemmap_populate_compound_
 					return -ENOMEM;
 				vmemmap_verify(pte, node, addr, addr + PAGE_SIZE);
 
-				/*
-				 * Populate the tail pages vmemmap page
-				 * It can fall in different pmd, hence
-				 * vmemmap_populate_address()
-				 */
-				pte = radix__vmemmap_populate_address(addr + PAGE_SIZE, node, NULL, NULL);
-				if (!pte)
-					return -ENOMEM;
-
-				next = addr + 2 * PAGE_SIZE;
+				next = addr + PAGE_SIZE;
 				continue;
 			}
 
--- a/include/linux/mm.h~mm-sparse-vmemmap-drop-the-extra-tail-page-from-device-dax-reservation
+++ a/include/linux/mm.h
@@ -5167,7 +5167,6 @@ static inline void vmem_altmap_free(stru
 }
 #endif
 
-#define VMEMMAP_RESERVE_NR	2
 #ifdef CONFIG_ARCH_WANT_OPTIMIZE_DAX_VMEMMAP
 static inline bool __vmemmap_can_optimize(struct vmem_altmap *altmap,
 					  struct dev_pagemap *pgmap)
@@ -5187,7 +5186,7 @@ static inline bool __vmemmap_can_optimiz
 	 * For vmemmap optimization with DAX we need minimum 2 vmemmap
 	 * pages. See layout diagram in Documentation/mm/vmemmap_dedup.rst
 	 */
-	return !altmap && (nr_vmemmap_pages > VMEMMAP_RESERVE_NR);
+	return !altmap && (nr_vmemmap_pages > VMEMMAP_OPTIMIZATION_PAGES);
 }
 /*
  * If we don't have an architecture override, use the generic rule
--- a/mm/mm_init.c~mm-sparse-vmemmap-drop-the-extra-tail-page-from-device-dax-reservation
+++ a/mm/mm_init.c
@@ -1056,7 +1056,7 @@ static inline unsigned long compound_nr_
 	if (!section_vmemmap_optimizable(ms))
 		return pgmap_vmemmap_nr(pgmap);
 
-	return VMEMMAP_RESERVE_NR * (PAGE_SIZE / sizeof(struct page));
+	return VMEMMAP_OPTIMIZATION_PAGES * (PAGE_SIZE / sizeof(struct page));
 }
 
 static void __ref memmap_init_compound(struct page *head,
--- a/mm/sparse-vmemmap.c~mm-sparse-vmemmap-drop-the-extra-tail-page-from-device-dax-reservation
+++ a/mm/sparse-vmemmap.c
@@ -136,7 +136,6 @@ int __meminit section_nr_vmemmap_pages(u
 {
 	const struct mem_section *ms = __pfn_to_section(pfn);
 	const int order = section_order(ms);
-	const int vmemmap_pages = pgmap ? VMEMMAP_RESERVE_NR : VMEMMAP_OPTIMIZATION_PAGES;
 	const unsigned long pages_per_compound = 1UL << order;
 
 	VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, PAGES_PER_SUBSECTION));
@@ -147,13 +146,13 @@ int __meminit section_nr_vmemmap_pages(u
 
 	if (order < PFN_SECTION_SHIFT) {
 		VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, pages_per_compound));
-		return vmemmap_pages * nr_pages / pages_per_compound;
+		return VMEMMAP_OPTIMIZATION_PAGES * nr_pages / pages_per_compound;
 	}
 
 	VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, PAGES_PER_SECTION));
 
 	if (IS_ALIGNED(pfn, pages_per_compound))
-		return vmemmap_pages;
+		return VMEMMAP_OPTIMIZATION_PAGES;
 
 	return 0;
 }
@@ -521,17 +520,11 @@ static int __meminit vmemmap_populate_co
 		if (!pte)
 			return -ENOMEM;
 
-		/* Populate the tail pages vmemmap page */
-		next = addr + PAGE_SIZE;
-		pte = vmemmap_populate_address(next, node, NULL, -1, flags);
-		if (!pte)
-			return -ENOMEM;
-
 		/*
 		 * Reuse the shared page for the rest of tail pages
 		 * See layout diagram in Documentation/mm/vmemmap_dedup.rst
 		 */
-		next += PAGE_SIZE;
+		next = addr + PAGE_SIZE;
 		rc = vmemmap_populate_range(next, last, node, NULL,
 					    page_to_pfn(page), flags);
 		if (rc)
_

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-section-orders.patch
mm-sparse-vmemmap-set-section-order-for-device-dax.patch
mm-sparse-vmemmap-switch-device-dax-to-shared-tail-vmemmap-pages.patch
mm-sparse-vmemmap-move-hvo-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