+ mm-sparse-vmemmap-set-compound-page-order-for-device-dax.patch added to mm-new branch

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

The patch titled
     Subject: mm/sparse-vmemmap: set compound page order for device DAX
has been added to the -mm mm-new branch.  Its filename is
     mm-sparse-vmemmap-set-compound-page-order-for-device-dax.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-set-compound-page-order-for-device-dax.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: set compound page order for device DAX
Date: Fri, 11 Sep 2026 13:02:22 +0800

Device DAX can use vmemmap optimization only when a full section is
populated with a compound-page geometry.  Record that geometry as the
compound page order in section metadata before populating the section, so
later vmemmap accounting and population decisions can use the section
state directly.

Clear the compound page order when the section becomes empty again.  Also
reject partial additions to a section that already has optimized vmemmap
mappings.  compound_nr_pages() determines how many struct pages to
initialize with a section as the smallest granularity.  A section
therefore cannot safely mix optimized and ordinary vmemmap layouts.

Partial additions continue to use ordinary vmemmap population, so they do
not save vmemmap memory.  Such additions are uncommon, and the lost saving
is negligible.

Link: https://lore.kernel.org/20260911050228.58884-6-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>
---

 mm/mm_init.c        |   15 +++++----------
 mm/sparse-vmemmap.c |   16 ++++++++++++----
 2 files changed, 17 insertions(+), 14 deletions(-)

--- a/mm/mm_init.c~mm-sparse-vmemmap-set-compound-page-order-for-device-dax
+++ a/mm/mm_init.c
@@ -1049,16 +1049,11 @@ static void zone_device_page_init_from_t
  * of an altmap. See vmemmap_populate_compound_pages().
  */
 static inline unsigned long compound_nr_pages(unsigned long pfn,
-					      struct vmem_altmap *altmap,
 					      struct dev_pagemap *pgmap)
 {
-	/*
-	 * If DAX memory is hot-plugged into an unoccupied subsection
-	 * of an early section, the unoptimized boot memmap is reused.
-	 * See section_activate().
-	 */
-	if (early_section(__pfn_to_section(pfn)) ||
-	    !vmemmap_can_optimize(altmap, pgmap))
+	const struct mem_section *ms = __pfn_to_section(pfn);
+
+	if (!section_vmemmap_optimizable(ms))
 		return pgmap_vmemmap_nr(pgmap);
 
 	return VMEMMAP_RESERVE_NR * (PAGE_SIZE / sizeof(struct page));
@@ -1144,7 +1139,7 @@ void __ref memmap_init_zone_device(struc
 	memcpy(&template, page, sizeof(*page));
 	if (pfns_per_compound != 1)
 		memmap_init_compound(page, pfn, zone_idx, nid, pgmap,
-				     compound_nr_pages(pfn, altmap, pgmap));
+				     compound_nr_pages(pfn, pgmap));
 	pfn += pfns_per_compound;
 
 	/* Initialize the remaining head pages from template. */
@@ -1160,7 +1155,7 @@ void __ref memmap_init_zone_device(struc
 			continue;
 
 		memmap_init_compound(page, pfn, zone_idx, nid, pgmap,
-				     compound_nr_pages(pfn, altmap, pgmap));
+				     compound_nr_pages(pfn, pgmap));
 	}
 
 	pageblock_migratetype_init_range(start_pfn, nr_pages, MIGRATE_MOVABLE,
--- a/mm/sparse-vmemmap.c~mm-sparse-vmemmap-set-compound-page-order-for-device-dax
+++ a/mm/sparse-vmemmap.c
@@ -135,14 +135,14 @@ int __meminit section_nr_vmemmap_pages(u
 		struct vmem_altmap *altmap, struct dev_pagemap *pgmap)
 {
 	const struct mem_section *ms = __pfn_to_section(pfn);
-	const int order = pgmap ? pgmap->vmemmap_shift : section_compound_order(ms);
+	const int order = section_compound_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));
 	VM_WARN_ON_ONCE(nr_pages > PAGES_PER_SECTION);
 
-	if (!vmemmap_can_optimize(altmap, pgmap) && !section_vmemmap_optimizable(ms))
+	if (!section_vmemmap_optimizable(ms))
 		return DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE);
 
 	if (order < PFN_SECTION_SHIFT) {
@@ -573,7 +573,7 @@ struct page * __meminit __populate_secti
 		!IS_ALIGNED(nr_pages, PAGES_PER_SUBSECTION)))
 		return NULL;
 
-	if (vmemmap_can_optimize(altmap, pgmap))
+	if (pgmap && section_vmemmap_optimizable(__pfn_to_section(pfn)))
 		r = vmemmap_populate_compound_pages(pfn, start, end, nid, pgmap);
 	else
 		r = vmemmap_populate(start, end, nid, altmap);
@@ -792,8 +792,10 @@ static void section_deactivate(unsigned
 	else if (memmap)
 		free_map_bootmem(memmap);
 
-	if (empty)
+	if (empty) {
 		ms->section_mem_map = (unsigned long)NULL;
+		section_set_compound_order(ms, 0);
+	}
 }
 
 static struct page * __meminit section_activate(int nid, unsigned long pfn,
@@ -803,8 +805,13 @@ static struct page * __meminit section_a
 	struct mem_section *ms = __pfn_to_section(pfn);
 	struct mem_section_usage *usage = NULL;
 	struct page *memmap;
+	unsigned int order;
 	int rc;
 
+	order = vmemmap_can_optimize(altmap, pgmap) ? pgmap->vmemmap_shift : 0;
+	if (nr_pages < PAGES_PER_SECTION && section_compound_order(ms))
+		return ERR_PTR(-EOPNOTSUPP);
+
 	if (!ms->usage) {
 		usage = kzalloc(mem_section_usage_size(), GFP_KERNEL);
 		if (!usage)
@@ -830,6 +837,7 @@ static struct page * __meminit section_a
 	if (nr_pages < PAGES_PER_SECTION && early_section(ms))
 		return pfn_to_page(pfn);
 
+	section_set_compound_order_range(pfn, nr_pages, order);
 	memmap = populate_section_memmap(pfn, nr_pages, nid, altmap, pgmap);
 	if (!memmap) {
 		section_deactivate(pfn, nr_pages, altmap, pgmap);
_

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