Some cleanups around memory hot(un)plug and SPARSEMEM. In essence,
we can limit CONFIG_MEMORY_HOTPLUG to CONFIG_SPARSEMEM_VMEMMAP,
remove some dead code, and move all the hotplug bits over to
mm/sparse-vmemmap.c.

Some further/related cleanups around other unnecessary code
(memory hole handling and complicated usemap allocation).

I have some further sparse.c cleanups lying around, and I'm planning
on getting rid of bootmem_info.c entirely.

Cross-compiled on a bunch of machines. Hot(un)plug tested with virtio-mem.

v1 -> v2:
* Added "mm/memory_hotplug: fix possible race in scan_movable_pages()"
* Update the comment above section_deactivate()
* Reordered the flags in sparse_init_one_section()
* Patch description improvements

---
David Hildenbrand (Arm) (15):
      mm/memory_hotplug: fix possible race in scan_movable_pages()
      mm/memory_hotplug: remove for_each_valid_pfn() usage
      mm/sparse: remove WARN_ONs from (online|offline)_mem_sections()
      mm/Kconfig: make CONFIG_MEMORY_HOTPLUG depend on CONFIG_SPARSEMEM_VMEMMAP
      mm/memory_hotplug: simplify check_pfn_span()
      mm/sparse: remove !CONFIG_SPARSEMEM_VMEMMAP leftovers for CONFIG_MEMORY_HOTPLUG
      mm/bootmem_info: remove handling for !CONFIG_SPARSEMEM_VMEMMAP
      mm/bootmem_info: avoid using sparse_decode_mem_map()
      mm/sparse: remove sparse_decode_mem_map()
      mm/sparse: remove CONFIG_MEMORY_HOTPLUG-specific usemap allocation handling
      mm: prepare to move subsection_map_init() to mm/sparse-vmemmap.c
      mm/sparse: drop set_section_nid() from sparse_add_section()
      mm/sparse: move sparse_init_one_section() to internal.h
      mm/sparse: move __section_mark_present() to internal.h
      mm/sparse: move memory hotplug bits to sparse-vmemmap.c

 include/linux/memory_hotplug.h |   2 -
 include/linux/mmzone.h         |   6 +-
 mm/Kconfig                     |   2 +-
 mm/bootmem_info.c              |  46 +---
 mm/internal.h                  |  47 ++++
 mm/memory_hotplug.c            |  35 ++-
 mm/mm_init.c                   |   2 +-
 mm/sparse-vmemmap.c            | 304 +++++++++++++++++++++++
 mm/sparse.c                    | 539 +----------------------------------------
 9 files changed, 377 insertions(+), 606 deletions(-)
---
base-commit: 3f4f1faa33544d0bd724e32980b6f211c3a9bc7b
change-id: 20260320-sparsemem_cleanups-ce4ddb2c47de

Best regards,
-- 
David Hildenbrand (Arm) <david@kernel.org>

On Fri, 20 Mar 2026 23:13:32 +0100 "David Hildenbrand (Arm)" <david@kernel.org> wrote:

> Some cleanups around memory hot(un)plug and SPARSEMEM. In essence,
> we can limit CONFIG_MEMORY_HOTPLUG to CONFIG_SPARSEMEM_VMEMMAP,
> remove some dead code, and move all the hotplug bits over to
> mm/sparse-vmemmap.c.
> 
> Some further/related cleanups around other unnecessary code
> (memory hole handling and complicated usemap allocation).

Sorry, for some (age-related) reason I've been sitting on v1 for nearly
a week.

Thanks, I've updated mm-unstable to this version.

Lorenzo, I've assumed that your conditional R-b on [01/15] is now
unconditional.

> v1 -> v2:
> * Added "mm/memory_hotplug: fix possible race in scan_movable_pages()"
> * Update the comment above section_deactivate()
> * Reordered the flags in sparse_init_one_section()
> * Patch description improvements

Here's how v2 altered mm.git:


 mm/internal.h       |    2 +-
 mm/memory_hotplug.c |   11 ++++++++---
 mm/sparse-vmemmap.c |    8 ++------
 3 files changed, 11 insertions(+), 10 deletions(-)

--- a/mm/internal.h~b
+++ a/mm/internal.h
@@ -985,7 +985,7 @@ static inline void sparse_init_one_secti
 
 	ms->section_mem_map &= ~SECTION_MAP_MASK;
 	ms->section_mem_map |= coded_mem_map;
-	ms->section_mem_map |= SECTION_HAS_MEM_MAP | flags;
+	ms->section_mem_map |= flags | SECTION_HAS_MEM_MAP;
 	ms->usage = usage;
 }
 
--- a/mm/memory_hotplug.c~b
+++ a/mm/memory_hotplug.c
@@ -1738,6 +1738,7 @@ static int scan_movable_pages(unsigned l
 	unsigned long pfn;
 
 	for (pfn = start; pfn < end; pfn++) {
+		unsigned long nr_pages;
 		struct page *page;
 		struct folio *folio;
 
@@ -1754,9 +1755,9 @@ static int scan_movable_pages(unsigned l
 		if (PageOffline(page) && page_count(page))
 			return -EBUSY;
 
-		if (!PageHuge(page))
-			continue;
 		folio = page_folio(page);
+		if (!folio_test_hugetlb(folio))
+			continue;
 		/*
 		 * This test is racy as we hold no reference or lock.  The
 		 * hugetlb page could have been free'ed and head is no longer
@@ -1766,7 +1767,11 @@ static int scan_movable_pages(unsigned l
 		 */
 		if (folio_test_hugetlb_migratable(folio))
 			goto found;
-		pfn |= folio_nr_pages(folio) - 1;
+		nr_pages = folio_nr_pages(folio);
+		if (unlikely(nr_pages < 1 || nr_pages > MAX_FOLIO_NR_PAGES ||
+			     !is_power_of_2(nr_pages)))
+			continue;
+		pfn |= nr_pages - 1;
 	}
 	return -ENOENT;
 found:
--- a/mm/sparse-vmemmap.c~b
+++ a/mm/sparse-vmemmap.c
@@ -725,11 +725,9 @@ static int fill_subsection_map(unsigned
 }
 
 /*
- * To deactivate a memory region, there are 3 cases to handle across
- * two configurations (SPARSEMEM_VMEMMAP={y,n}):
+ * To deactivate a memory region, there are 3 cases to handle:
  *
- * 1. deactivation of a partial hot-added section (only possible in
- *    the SPARSEMEM_VMEMMAP=y case).
+ * 1. deactivation of a partial hot-added section:
  *      a) section was present at memory init.
  *      b) section was hot-added post memory init.
  * 2. deactivation of a complete hot-added section.
@@ -737,8 +735,6 @@ static int fill_subsection_map(unsigned
  *
  * For 1, when subsection_map does not empty we will not be freeing the
  * usage map, but still need to free the vmemmap range.
- *
- * For 2 and 3, the SPARSEMEM_VMEMMAP={y,n} cases are unified
  */
 static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
 		struct vmem_altmap *altmap)
_

On Thu, Mar 26, 2026 at 07:58:16PM -0700, Andrew Morton wrote:
> Lorenzo, I've assumed that your conditional R-b on [01/15] is now
> unconditional.

Yeah that's fine.

Cheers, Lorenzo