[PATCHv5 00/17] mm: Eliminate fake head pages from vmemmap optimization

[PATCHv5 00/17] mm: Eliminate fake head pages from vmemmap optimization

[Kiryl Shutsemau]

This series removes "fake head pages" from the HugeTLB vmemmap
optimization (HVO) by changing how tail pages encode their relationship
to the head page.

It simplifies compound_head() and page_ref_add_unless(). Both are in the
hot path.

Background
==========

HVO reduces memory overhead by freeing vmemmap pages for HugeTLB pages
and remapping the freed virtual addresses to a single physical page.
Previously, all tail page vmemmap entries were remapped to the first
vmemmap page (containing the head struct page), creating "fake heads" -
tail pages that appear to have PG_head set when accessed through the
deduplicated vmemmap.

This required special handling in compound_head() to detect and work
around fake heads, adding complexity and overhead to a very hot path.

New Approach
============

For architectures/configs where sizeof(struct page) is a power of 2 (the
common case), this series changes how position of the head page is encoded
in the tail pages.

Instead of storing a pointer to the head page, the ->compound_info
(renamed from ->compound_head) now stores a mask.

The mask can be applied to any tail page's virtual address to compute
the head page address. Critically, all tail pages of the same order now
have identical compound_info values, regardless of which compound page
they belong to.

The key insight is that all tail pages of the same order now have
identical compound_info values, regardless of which compound page they
belong to. This allows a single page of tail struct pages to be shared
across all huge pages of the same order on a NUMA node.

Benefits
========

1. Simplified compound_head(): No fake head detection needed, can be
   implemented in a branchless manner.

2. Simplified page_ref_add_unless(): RCU protection removed since there's
   no race with fake head remapping.

3. Cleaner architecture: The shared tail pages are truly read-only and
   contain valid tail page metadata.

If sizeof(struct page) is not power-of-2, there are no functional changes.
HVO is not supported in this configuration.

I had hoped to see performance improvement, but my testing thus far has
shown either no change or only a slight improvement within the noise.

Series Organization
===================

Patch 1: Preparation - move MAX_FOLIO_ORDER to mmzone.h
Patches 2-4: Refactoring - interface changes, field rename, code movement
Patches 5-6: Arch fixes - align vmemmap for riscv and LoongArch
Patch 7: Core change - new mask-based compound_head() encoding
Patch 8: Correctness fix - page_zonenum() must use head page
Patch 9: Add memmap alignment check for compound_info_has_mask()
Patch 10: Refactor vmemmap_walk for new design
Patch 11: Eliminate fake heads with shared tail pages
Patches 12-15: Cleanup - remove fake head infrastructure
Patch 16: Documentation update
Patch 17: Get rid of opencoded compound_head() in page_slab()

Changes in v5:
==============
  - Rebased to mm-everything-2026-01-27-04-35

  - Add arch-specific patches to align vmemmap to maximal folio size
    for riscv and LoongArch architectures.

  - Strengthen the memmap alignment check in mm/sparse.c: use BUG()
    for CONFIG_DEBUG_VM, WARN() otherwise. (Muchun)

  - Use cmpxchg() instead of hugetlb_lock to update vmemmap_tails
    array. (Muchun)

  - Update page_slab().

Changes in v4:
==============
  - Fix build issues due to linux/mmzone.h <-> linux/pgtable.h
    dependency loop by avoiding including linux/pgtable.h into
    linux/mmzone.h

  - Rework vmemmap_remap_alloc() interface. (Muchun)

  - Use &folio->page instead of folio address for optimization
    target. (Muchun)

Changes in v3:
==============
  - Fixed error recovery path in vmemmap_remap_free() to pass correct start
    address for TLB flush. (Muchun)

  - Wrapped the mask-based compound_info encoding within CONFIG_SPARSEMEM_VMEMMAP
    check via compound_info_has_mask(). For other memory models, alignment
    guarantees are harder to verify. (Muchun)

  - Updated vmemmap_dedup.rst documentation wording: changed "vmemmap_tail
    shared for the struct hstate" to "A single, per-node page frame shared
    among all hugepages of the same size". (Muchun)

  - Fixed build error with MAX_FOLIO_ORDER expanding to undefined PUD_ORDER
    in certain configurations. (kernel test robot)

Changes in v2:
==============

- Handle boot-allocated huge pages correctly. (Frank)

- Changed from per-hstate vmemmap_tail to per-node vmemmap_tails[] array
  in pglist_data. (Muchun)

- Added spin_lock(&hugetlb_lock) protection in vmemmap_get_tail() to fix
  a race condition where two threads could both allocate tail pages.
  The losing thread now properly frees its allocated page. (Usama)

- Add warning if memmap is not aligned to MAX_FOLIO_SIZE, which is
  required for the mask approach. (Muchun)

- Make page_zonenum() use head page - correctness fix since shared
  tail pages cannot have valid zone information. (Muchun)

- Added 'const' qualifier to head parameter in set_compound_head() and
  prep_compound_tail(). (Usama)

- Updated commit messages.


Kiryl Shutsemau (17):
  mm: Move MAX_FOLIO_ORDER definition to mmzone.h
  mm: Change the interface of prep_compound_tail()
  mm: Rename the 'compound_head' field in the 'struct page' to
    'compound_info'
  mm: Move set/clear_compound_head() next to compound_head()
  riscv/mm: Align vmemmap to maximal folio size
  LoongArch/mm: Align vmemmap to maximal folio size
  mm: Rework compound_head() for power-of-2 sizeof(struct page)
  mm: Make page_zonenum() use head page
  mm/sparse: Check memmap alignment for compound_info_has_mask()
  mm/hugetlb: Refactor code around vmemmap_walk
  mm/hugetlb: Remove fake head pages
  mm: Drop fake head checks
  hugetlb: Remove VMEMMAP_SYNCHRONIZE_RCU
  mm/hugetlb: Remove hugetlb_optimize_vmemmap_key static key
  mm: Remove the branch from compound_head()
  hugetlb: Update vmemmap_dedup.rst
  mm/slab: Use compound_head() in page_slab()

 .../admin-guide/kdump/vmcoreinfo.rst          |   2 +-
 Documentation/mm/vmemmap_dedup.rst            |  62 ++--
 arch/loongarch/include/asm/pgtable.h          |   3 +-
 arch/riscv/mm/init.c                          |   3 +-
 include/linux/mm.h                            |  31 --
 include/linux/mm_types.h                      |  20 +-
 include/linux/mmzone.h                        |  46 +++
 include/linux/page-flags.h                    | 167 +++++-----
 include/linux/page_ref.h                      |   8 +-
 include/linux/types.h                         |   2 +-
 kernel/vmcore_info.c                          |   2 +-
 mm/hugetlb.c                                  |   8 +-
 mm/hugetlb_vmemmap.c                          | 290 ++++++++----------
 mm/internal.h                                 |  12 +-
 mm/mm_init.c                                  |   2 +-
 mm/page_alloc.c                               |   4 +-
 mm/slab.h                                     |   8 +-
 mm/sparse-vmemmap.c                           |  44 ++-
 mm/sparse.c                                   |  13 +
 mm/util.c                                     |  16 +-
 20 files changed, 371 insertions(+), 372 deletions(-)

-- 
2.51.2

[PATCHv5 01/17] mm: Move MAX_FOLIO_ORDER definition to mmzone.h

[Kiryl Shutsemau]

Move MAX_FOLIO_ORDER definition from mm.h to mmzone.h.

This is preparation for adding the vmemmap_tails array to struct
pglist_data, which requires MAX_FOLIO_ORDER to be available in mmzone.h.

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Acked-by: Zi Yan <ziy@nvidia.com>
Acked-by: Muchun Song <muchun.song@linux.dev>
---
 include/linux/mm.h     | 31 -------------------------------
 include/linux/mmzone.h | 31 +++++++++++++++++++++++++++++++
 2 files changed, 31 insertions(+), 31 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index f8a8fd47399c..8d5fa655fea4 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -27,7 +27,6 @@
 #include <linux/page-flags.h>
 #include <linux/page_ref.h>
 #include <linux/overflow.h>
-#include <linux/sizes.h>
 #include <linux/sched.h>
 #include <linux/pgtable.h>
 #include <linux/kasan.h>
@@ -2477,36 +2476,6 @@ static inline unsigned long folio_nr_pages(const struct folio *folio)
 	return folio_large_nr_pages(folio);
 }
 
-#if !defined(CONFIG_HAVE_GIGANTIC_FOLIOS)
-/*
- * We don't expect any folios that exceed buddy sizes (and consequently
- * memory sections).
- */
-#define MAX_FOLIO_ORDER		MAX_PAGE_ORDER
-#elif defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
-/*
- * Only pages within a single memory section are guaranteed to be
- * contiguous. By limiting folios to a single memory section, all folio
- * pages are guaranteed to be contiguous.
- */
-#define MAX_FOLIO_ORDER		PFN_SECTION_SHIFT
-#elif defined(CONFIG_HUGETLB_PAGE)
-/*
- * There is no real limit on the folio size. We limit them to the maximum we
- * currently expect (see CONFIG_HAVE_GIGANTIC_FOLIOS): with hugetlb, we expect
- * no folios larger than 16 GiB on 64bit and 1 GiB on 32bit.
- */
-#define MAX_FOLIO_ORDER		get_order(IS_ENABLED(CONFIG_64BIT) ? SZ_16G : SZ_1G)
-#else
-/*
- * Without hugetlb, gigantic folios that are bigger than a single PUD are
- * currently impossible.
- */
-#define MAX_FOLIO_ORDER		PUD_ORDER
-#endif
-
-#define MAX_FOLIO_NR_PAGES	(1UL << MAX_FOLIO_ORDER)
-
 /*
  * compound_nr() returns the number of pages in this potentially compound
  * page.  compound_nr() can be called on a tail page, and is defined to
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 3e51190a55e4..be8ce40b5638 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -23,6 +23,7 @@
 #include <linux/page-flags.h>
 #include <linux/local_lock.h>
 #include <linux/zswap.h>
+#include <linux/sizes.h>
 #include <asm/page.h>
 
 /* Free memory management - zoned buddy allocator.  */
@@ -61,6 +62,36 @@
  */
 #define PAGE_ALLOC_COSTLY_ORDER 3
 
+#if !defined(CONFIG_HAVE_GIGANTIC_FOLIOS)
+/*
+ * We don't expect any folios that exceed buddy sizes (and consequently
+ * memory sections).
+ */
+#define MAX_FOLIO_ORDER		MAX_PAGE_ORDER
+#elif defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
+/*
+ * Only pages within a single memory section are guaranteed to be
+ * contiguous. By limiting folios to a single memory section, all folio
+ * pages are guaranteed to be contiguous.
+ */
+#define MAX_FOLIO_ORDER		PFN_SECTION_SHIFT
+#elif defined(CONFIG_HUGETLB_PAGE)
+/*
+ * There is no real limit on the folio size. We limit them to the maximum we
+ * currently expect (see CONFIG_HAVE_GIGANTIC_FOLIOS): with hugetlb, we expect
+ * no folios larger than 16 GiB on 64bit and 1 GiB on 32bit.
+ */
+#define MAX_FOLIO_ORDER		get_order(IS_ENABLED(CONFIG_64BIT) ? SZ_16G : SZ_1G)
+#else
+/*
+ * Without hugetlb, gigantic folios that are bigger than a single PUD are
+ * currently impossible.
+ */
+#define MAX_FOLIO_ORDER		PUD_ORDER
+#endif
+
+#define MAX_FOLIO_NR_PAGES	(1UL << MAX_FOLIO_ORDER)
+
 enum migratetype {
 	MIGRATE_UNMOVABLE,
 	MIGRATE_MOVABLE,
-- 
2.51.2

[PATCHv5 02/17] mm: Change the interface of prep_compound_tail()

[Kiryl Shutsemau]

Instead of passing down the head page and tail page index, pass the tail
and head pages directly, as well as the order of the compound page.

This is a preparation for changing how the head position is encoded in
the tail page.

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Zi Yan <ziy@nvidia.com>
---
 include/linux/page-flags.h |  4 +++-
 mm/hugetlb.c               |  8 +++++---
 mm/internal.h              | 12 ++++++------
 mm/mm_init.c               |  2 +-
 mm/page_alloc.c            |  2 +-
 5 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index f7a0e4af0c73..8a3694369e15 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -865,7 +865,9 @@ static inline bool folio_test_large(const struct folio *folio)
 	return folio_test_head(folio);
 }
 
-static __always_inline void set_compound_head(struct page *page, struct page *head)
+static __always_inline void set_compound_head(struct page *page,
+					      const struct page *head,
+					      unsigned int order)
 {
 	WRITE_ONCE(page->compound_head, (unsigned long)head + 1);
 }
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 6e855a32de3d..54ba7cd05a86 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3168,6 +3168,7 @@ int __alloc_bootmem_huge_page(struct hstate *h, int nid)
 
 /* Initialize [start_page:end_page_number] tail struct pages of a hugepage */
 static void __init hugetlb_folio_init_tail_vmemmap(struct folio *folio,
+					struct hstate *h,
 					unsigned long start_page_number,
 					unsigned long end_page_number)
 {
@@ -3176,6 +3177,7 @@ static void __init hugetlb_folio_init_tail_vmemmap(struct folio *folio,
 	struct page *page = folio_page(folio, start_page_number);
 	unsigned long head_pfn = folio_pfn(folio);
 	unsigned long pfn, end_pfn = head_pfn + end_page_number;
+	unsigned int order = huge_page_order(h);
 
 	/*
 	 * As we marked all tail pages with memblock_reserved_mark_noinit(),
@@ -3183,7 +3185,7 @@ static void __init hugetlb_folio_init_tail_vmemmap(struct folio *folio,
 	 */
 	for (pfn = head_pfn + start_page_number; pfn < end_pfn; page++, pfn++) {
 		__init_single_page(page, pfn, zone, nid);
-		prep_compound_tail((struct page *)folio, pfn - head_pfn);
+		prep_compound_tail(page, &folio->page, order);
 		set_page_count(page, 0);
 	}
 }
@@ -3203,7 +3205,7 @@ static void __init hugetlb_folio_init_vmemmap(struct folio *folio,
 	__folio_set_head(folio);
 	ret = folio_ref_freeze(folio, 1);
 	VM_BUG_ON(!ret);
-	hugetlb_folio_init_tail_vmemmap(folio, 1, nr_pages);
+	hugetlb_folio_init_tail_vmemmap(folio, h, 1, nr_pages);
 	prep_compound_head(&folio->page, huge_page_order(h));
 }
 
@@ -3260,7 +3262,7 @@ static void __init prep_and_add_bootmem_folios(struct hstate *h,
 			 * time as this is early in boot and there should
 			 * be no contention.
 			 */
-			hugetlb_folio_init_tail_vmemmap(folio,
+			hugetlb_folio_init_tail_vmemmap(folio, h,
 					HUGETLB_VMEMMAP_RESERVE_PAGES,
 					pages_per_huge_page(h));
 		}
diff --git a/mm/internal.h b/mm/internal.h
index d67e8bb75734..037ddcda25ff 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -879,13 +879,13 @@ static inline void prep_compound_head(struct page *page, unsigned int order)
 		INIT_LIST_HEAD(&folio->_deferred_list);
 }
 
-static inline void prep_compound_tail(struct page *head, int tail_idx)
+static inline void prep_compound_tail(struct page *tail,
+				      const struct page *head,
+				      unsigned int order)
 {
-	struct page *p = head + tail_idx;
-
-	p->mapping = TAIL_MAPPING;
-	set_compound_head(p, head);
-	set_page_private(p, 0);
+	tail->mapping = TAIL_MAPPING;
+	set_compound_head(tail, head, order);
+	set_page_private(tail, 0);
 }
 
 void post_alloc_hook(struct page *page, unsigned int order, gfp_t gfp_flags);
diff --git a/mm/mm_init.c b/mm/mm_init.c
index 1a29a719af58..ba50f4c4337b 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -1099,7 +1099,7 @@ static void __ref memmap_init_compound(struct page *head,
 		struct page *page = pfn_to_page(pfn);
 
 		__init_zone_device_page(page, pfn, zone_idx, nid, pgmap);
-		prep_compound_tail(head, pfn - head_pfn);
+		prep_compound_tail(page, head, order);
 		set_page_count(page, 0);
 	}
 	prep_compound_head(head, order);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e4104973e22f..00c7ea958767 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -744,7 +744,7 @@ void prep_compound_page(struct page *page, unsigned int order)
 
 	__SetPageHead(page);
 	for (i = 1; i < nr_pages; i++)
-		prep_compound_tail(page, i);
+		prep_compound_tail(page + i, page, order);
 
 	prep_compound_head(page, order);
 }
-- 
2.51.2

[PATCHv5 03/17] mm: Rename the 'compound_head' field in the 'struct page' to 'compound_info'

[Kiryl Shutsemau]

The 'compound_head' field in the 'struct page' encodes whether the page
is a tail and where to locate the head page. Bit 0 is set if the page is
a tail, and the remaining bits in the field point to the head page.

As preparation for changing how the field encodes information about the
head page, rename the field to 'compound_info'.

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Zi Yan <ziy@nvidia.com>
---
 .../admin-guide/kdump/vmcoreinfo.rst          |  2 +-
 Documentation/mm/vmemmap_dedup.rst            |  6 +++---
 include/linux/mm_types.h                      | 20 +++++++++----------
 include/linux/page-flags.h                    | 18 ++++++++---------
 include/linux/types.h                         |  2 +-
 kernel/vmcore_info.c                          |  2 +-
 mm/page_alloc.c                               |  2 +-
 mm/slab.h                                     |  2 +-
 mm/util.c                                     |  2 +-
 9 files changed, 28 insertions(+), 28 deletions(-)

diff --git a/Documentation/admin-guide/kdump/vmcoreinfo.rst b/Documentation/admin-guide/kdump/vmcoreinfo.rst
index 404a15f6782c..7663c610fe90 100644
--- a/Documentation/admin-guide/kdump/vmcoreinfo.rst
+++ b/Documentation/admin-guide/kdump/vmcoreinfo.rst
@@ -141,7 +141,7 @@ nodemask_t
 The size of a nodemask_t type. Used to compute the number of online
 nodes.
 
-(page, flags|_refcount|mapping|lru|_mapcount|private|compound_order|compound_head)
+(page, flags|_refcount|mapping|lru|_mapcount|private|compound_order|compound_info)
 ----------------------------------------------------------------------------------
 
 User-space tools compute their values based on the offset of these
diff --git a/Documentation/mm/vmemmap_dedup.rst b/Documentation/mm/vmemmap_dedup.rst
index b4a55b6569fa..1863d88d2dcb 100644
--- a/Documentation/mm/vmemmap_dedup.rst
+++ b/Documentation/mm/vmemmap_dedup.rst
@@ -24,7 +24,7 @@ For each base page, there is a corresponding ``struct page``.
 Within the HugeTLB subsystem, only the first 4 ``struct page`` are used to
 contain unique information about a HugeTLB page. ``__NR_USED_SUBPAGE`` provides
 this upper limit. The only 'useful' information in the remaining ``struct page``
-is the compound_head field, and this field is the same for all tail pages.
+is the compound_info field, and this field is the same for all tail pages.
 
 By removing redundant ``struct page`` for HugeTLB pages, memory can be returned
 to the buddy allocator for other uses.
@@ -124,10 +124,10 @@ Here is how things look before optimization::
  |           |
  +-----------+
 
-The value of page->compound_head is the same for all tail pages. The first
+The value of page->compound_info is the same for all tail pages. The first
 page of ``struct page`` (page 0) associated with the HugeTLB page contains the 4
 ``struct page`` necessary to describe the HugeTLB. The only use of the remaining
-pages of ``struct page`` (page 1 to page 7) is to point to page->compound_head.
+pages of ``struct page`` (page 1 to page 7) is to point to page->compound_info.
 Therefore, we can remap pages 1 to 7 to page 0. Only 1 page of ``struct page``
 will be used for each HugeTLB page. This will allow us to free the remaining
 7 pages to the buddy allocator.
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 3cc8ae722886..7bc82a2b889f 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -126,14 +126,14 @@ struct page {
 			atomic_long_t pp_ref_count;
 		};
 		struct {	/* Tail pages of compound page */
-			unsigned long compound_head;	/* Bit zero is set */
+			unsigned long compound_info;	/* Bit zero is set */
 		};
 		struct {	/* ZONE_DEVICE pages */
 			/*
-			 * The first word is used for compound_head or folio
+			 * The first word is used for compound_info or folio
 			 * pgmap
 			 */
-			void *_unused_pgmap_compound_head;
+			void *_unused_pgmap_compound_info;
 			void *zone_device_data;
 			/*
 			 * ZONE_DEVICE private pages are counted as being
@@ -409,7 +409,7 @@ struct folio {
 	/* private: avoid cluttering the output */
 				/* For the Unevictable "LRU list" slot */
 				struct {
-					/* Avoid compound_head */
+					/* Avoid compound_info */
 					void *__filler;
 	/* public: */
 					unsigned int mlock_count;
@@ -510,7 +510,7 @@ struct folio {
 FOLIO_MATCH(flags, flags);
 FOLIO_MATCH(lru, lru);
 FOLIO_MATCH(mapping, mapping);
-FOLIO_MATCH(compound_head, lru);
+FOLIO_MATCH(compound_info, lru);
 FOLIO_MATCH(__folio_index, index);
 FOLIO_MATCH(private, private);
 FOLIO_MATCH(_mapcount, _mapcount);
@@ -529,7 +529,7 @@ FOLIO_MATCH(_last_cpupid, _last_cpupid);
 	static_assert(offsetof(struct folio, fl) ==			\
 			offsetof(struct page, pg) + sizeof(struct page))
 FOLIO_MATCH(flags, _flags_1);
-FOLIO_MATCH(compound_head, _head_1);
+FOLIO_MATCH(compound_info, _head_1);
 FOLIO_MATCH(_mapcount, _mapcount_1);
 FOLIO_MATCH(_refcount, _refcount_1);
 #undef FOLIO_MATCH
@@ -537,13 +537,13 @@ FOLIO_MATCH(_refcount, _refcount_1);
 	static_assert(offsetof(struct folio, fl) ==			\
 			offsetof(struct page, pg) + 2 * sizeof(struct page))
 FOLIO_MATCH(flags, _flags_2);
-FOLIO_MATCH(compound_head, _head_2);
+FOLIO_MATCH(compound_info, _head_2);
 #undef FOLIO_MATCH
 #define FOLIO_MATCH(pg, fl)						\
 	static_assert(offsetof(struct folio, fl) ==			\
 			offsetof(struct page, pg) + 3 * sizeof(struct page))
 FOLIO_MATCH(flags, _flags_3);
-FOLIO_MATCH(compound_head, _head_3);
+FOLIO_MATCH(compound_info, _head_3);
 #undef FOLIO_MATCH
 
 /**
@@ -609,8 +609,8 @@ struct ptdesc {
 #define TABLE_MATCH(pg, pt)						\
 	static_assert(offsetof(struct page, pg) == offsetof(struct ptdesc, pt))
 TABLE_MATCH(flags, pt_flags);
-TABLE_MATCH(compound_head, pt_list);
-TABLE_MATCH(compound_head, _pt_pad_1);
+TABLE_MATCH(compound_info, pt_list);
+TABLE_MATCH(compound_info, _pt_pad_1);
 TABLE_MATCH(mapping, __page_mapping);
 TABLE_MATCH(__folio_index, pt_index);
 TABLE_MATCH(rcu_head, pt_rcu_head);
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 8a3694369e15..aa46d49e82f7 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -213,7 +213,7 @@ static __always_inline const struct page *page_fixed_fake_head(const struct page
 	/*
 	 * Only addresses aligned with PAGE_SIZE of struct page may be fake head
 	 * struct page. The alignment check aims to avoid access the fields (
-	 * e.g. compound_head) of the @page[1]. It can avoid touch a (possibly)
+	 * e.g. compound_info) of the @page[1]. It can avoid touch a (possibly)
 	 * cold cacheline in some cases.
 	 */
 	if (IS_ALIGNED((unsigned long)page, PAGE_SIZE) &&
@@ -223,7 +223,7 @@ static __always_inline const struct page *page_fixed_fake_head(const struct page
 		 * because the @page is a compound page composed with at least
 		 * two contiguous pages.
 		 */
-		unsigned long head = READ_ONCE(page[1].compound_head);
+		unsigned long head = READ_ONCE(page[1].compound_info);
 
 		if (likely(head & 1))
 			return (const struct page *)(head - 1);
@@ -281,7 +281,7 @@ static __always_inline int page_is_fake_head(const struct page *page)
 
 static __always_inline unsigned long _compound_head(const struct page *page)
 {
-	unsigned long head = READ_ONCE(page->compound_head);
+	unsigned long head = READ_ONCE(page->compound_info);
 
 	if (unlikely(head & 1))
 		return head - 1;
@@ -320,13 +320,13 @@ static __always_inline unsigned long _compound_head(const struct page *page)
 
 static __always_inline int PageTail(const struct page *page)
 {
-	return READ_ONCE(page->compound_head) & 1 || page_is_fake_head(page);
+	return READ_ONCE(page->compound_info) & 1 || page_is_fake_head(page);
 }
 
 static __always_inline int PageCompound(const struct page *page)
 {
 	return test_bit(PG_head, &page->flags.f) ||
-	       READ_ONCE(page->compound_head) & 1;
+	       READ_ONCE(page->compound_info) & 1;
 }
 
 #define	PAGE_POISON_PATTERN	-1l
@@ -348,7 +348,7 @@ static const unsigned long *const_folio_flags(const struct folio *folio,
 {
 	const struct page *page = &folio->page;
 
-	VM_BUG_ON_PGFLAGS(page->compound_head & 1, page);
+	VM_BUG_ON_PGFLAGS(page->compound_info & 1, page);
 	VM_BUG_ON_PGFLAGS(n > 0 && !test_bit(PG_head, &page->flags.f), page);
 	return &page[n].flags.f;
 }
@@ -357,7 +357,7 @@ static unsigned long *folio_flags(struct folio *folio, unsigned n)
 {
 	struct page *page = &folio->page;
 
-	VM_BUG_ON_PGFLAGS(page->compound_head & 1, page);
+	VM_BUG_ON_PGFLAGS(page->compound_info & 1, page);
 	VM_BUG_ON_PGFLAGS(n > 0 && !test_bit(PG_head, &page->flags.f), page);
 	return &page[n].flags.f;
 }
@@ -869,12 +869,12 @@ static __always_inline void set_compound_head(struct page *page,
 					      const struct page *head,
 					      unsigned int order)
 {
-	WRITE_ONCE(page->compound_head, (unsigned long)head + 1);
+	WRITE_ONCE(page->compound_info, (unsigned long)head + 1);
 }
 
 static __always_inline void clear_compound_head(struct page *page)
 {
-	WRITE_ONCE(page->compound_head, 0);
+	WRITE_ONCE(page->compound_info, 0);
 }
 
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
diff --git a/include/linux/types.h b/include/linux/types.h
index f69be881369f..604697abf151 100644
--- a/include/linux/types.h
+++ b/include/linux/types.h
@@ -234,7 +234,7 @@ struct ustat {
  *
  * This guarantee is important for few reasons:
  *  - future call_rcu_lazy() will make use of lower bits in the pointer;
- *  - the structure shares storage space in struct page with @compound_head,
+ *  - the structure shares storage space in struct page with @compound_info,
  *    which encode PageTail() in bit 0. The guarantee is needed to avoid
  *    false-positive PageTail().
  */
diff --git a/kernel/vmcore_info.c b/kernel/vmcore_info.c
index 46198580373a..0a46df3e3db9 100644
--- a/kernel/vmcore_info.c
+++ b/kernel/vmcore_info.c
@@ -198,7 +198,7 @@ static int __init crash_save_vmcoreinfo_init(void)
 	VMCOREINFO_OFFSET(page, lru);
 	VMCOREINFO_OFFSET(page, _mapcount);
 	VMCOREINFO_OFFSET(page, private);
-	VMCOREINFO_OFFSET(page, compound_head);
+	VMCOREINFO_OFFSET(page, compound_info);
 	VMCOREINFO_OFFSET(pglist_data, node_zones);
 	VMCOREINFO_OFFSET(pglist_data, nr_zones);
 #ifdef CONFIG_FLATMEM
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 00c7ea958767..cb7375eb1713 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -731,7 +731,7 @@ static inline bool pcp_allowed_order(unsigned int order)
  * The first PAGE_SIZE page is called the "head page" and have PG_head set.
  *
  * The remaining PAGE_SIZE pages are called "tail pages". PageTail() is encoded
- * in bit 0 of page->compound_head. The rest of bits is pointer to head page.
+ * in bit 0 of page->compound_info. The rest of bits is pointer to head page.
  *
  * The first tail page's ->compound_order holds the order of allocation.
  * This usage means that zero-order pages may not be compound.
diff --git a/mm/slab.h b/mm/slab.h
index e767aa7e91b0..8a2a9c6c697b 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -100,7 +100,7 @@ struct slab {
 #define SLAB_MATCH(pg, sl)						\
 	static_assert(offsetof(struct page, pg) == offsetof(struct slab, sl))
 SLAB_MATCH(flags, flags);
-SLAB_MATCH(compound_head, slab_cache);	/* Ensure bit 0 is clear */
+SLAB_MATCH(compound_info, slab_cache);	/* Ensure bit 0 is clear */
 SLAB_MATCH(_refcount, __page_refcount);
 #ifdef CONFIG_MEMCG
 SLAB_MATCH(memcg_data, obj_exts);
diff --git a/mm/util.c b/mm/util.c
index b05ab6f97e11..3ebcb9e6035c 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1247,7 +1247,7 @@ void snapshot_page(struct page_snapshot *ps, const struct page *page)
 again:
 	memset(&ps->folio_snapshot, 0, sizeof(struct folio));
 	memcpy(&ps->page_snapshot, page, sizeof(*page));
-	head = ps->page_snapshot.compound_head;
+	head = ps->page_snapshot.compound_info;
 	if ((head & 1) == 0) {
 		ps->idx = 0;
 		foliop = (struct folio *)&ps->page_snapshot;
-- 
2.51.2

[PATCHv5 04/17] mm: Move set/clear_compound_head() next to compound_head()

[Kiryl Shutsemau]

Move set_compound_head() and clear_compound_head() to be adjacent to the
compound_head() function in page-flags.h.

These functions encode and decode the same compound_info field, so
keeping them together makes it easier to verify their logic is
consistent, especially when the encoding changes.

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Zi Yan <ziy@nvidia.com>
---
 include/linux/page-flags.h | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index aa46d49e82f7..d14a17ffb55b 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -290,6 +290,18 @@ static __always_inline unsigned long _compound_head(const struct page *page)
 
 #define compound_head(page)	((typeof(page))_compound_head(page))
 
+static __always_inline void set_compound_head(struct page *page,
+					      const struct page *head,
+					      unsigned int order)
+{
+	WRITE_ONCE(page->compound_info, (unsigned long)head + 1);
+}
+
+static __always_inline void clear_compound_head(struct page *page)
+{
+	WRITE_ONCE(page->compound_info, 0);
+}
+
 /**
  * page_folio - Converts from page to folio.
  * @p: The page.
@@ -865,18 +877,6 @@ static inline bool folio_test_large(const struct folio *folio)
 	return folio_test_head(folio);
 }
 
-static __always_inline void set_compound_head(struct page *page,
-					      const struct page *head,
-					      unsigned int order)
-{
-	WRITE_ONCE(page->compound_info, (unsigned long)head + 1);
-}
-
-static __always_inline void clear_compound_head(struct page *page)
-{
-	WRITE_ONCE(page->compound_info, 0);
-}
-
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 static inline void ClearPageCompound(struct page *page)
 {
-- 
2.51.2

[PATCHv5 05/17] riscv/mm: Align vmemmap to maximal folio size

[Kiryl Shutsemau]

The upcoming change to the HugeTLB vmemmap optimization (HVO) requires
struct pages of the head page to be naturally aligned with regard to the
folio size.

Align vmemmap to MAX_FOLIO_NR_PAGES.

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
---
 arch/riscv/mm/init.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 21d534824624..c555b9a4fdce 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -63,7 +63,8 @@ phys_addr_t phys_ram_base __ro_after_init;
 EXPORT_SYMBOL(phys_ram_base);
 
 #ifdef CONFIG_SPARSEMEM_VMEMMAP
-#define VMEMMAP_ADDR_ALIGN	(1ULL << SECTION_SIZE_BITS)
+#define VMEMMAP_ADDR_ALIGN	max(1ULL << SECTION_SIZE_BITS, \
+				    MAX_FOLIO_NR_PAGES * sizeof(struct page))
 
 unsigned long vmemmap_start_pfn __ro_after_init;
 EXPORT_SYMBOL(vmemmap_start_pfn);
-- 
2.51.2

[PATCHv5 06/17] LoongArch/mm: Align vmemmap to maximal folio size

[Kiryl Shutsemau]

The upcoming change to the HugeTLB vmemmap optimization (HVO) requires
struct pages of the head page to be naturally aligned with regard to the
folio size.

Align vmemmap to MAX_FOLIO_NR_PAGES.

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
---
 arch/loongarch/include/asm/pgtable.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/loongarch/include/asm/pgtable.h b/arch/loongarch/include/asm/pgtable.h
index c33b3bcb733e..f9416acb9156 100644
--- a/arch/loongarch/include/asm/pgtable.h
+++ b/arch/loongarch/include/asm/pgtable.h
@@ -113,7 +113,8 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)];
 	 min(PTRS_PER_PGD * PTRS_PER_PUD * PTRS_PER_PMD * PTRS_PER_PTE * PAGE_SIZE, (1UL << cpu_vabits) / 2) - PMD_SIZE - VMEMMAP_SIZE - KFENCE_AREA_SIZE)
 #endif
 
-#define vmemmap		((struct page *)((VMALLOC_END + PMD_SIZE) & PMD_MASK))
+#define VMEMMAP_ALIGN	max(PMD_SIZE, MAX_FOLIO_NR_PAGES * sizeof(struct page))
+#define vmemmap		((struct page *)(ALIGN(VMALLOC_END, VMEMMAP_ALIGN)))
 #define VMEMMAP_END	((unsigned long)vmemmap + VMEMMAP_SIZE - 1)
 
 #define KFENCE_AREA_START	(VMEMMAP_END + 1)
-- 
2.51.2

[PATCHv5 07/17] mm: Rework compound_head() for power-of-2 sizeof(struct page)

[Kiryl Shutsemau]

For tail pages, the kernel uses the 'compound_info' field to get to the
head page. The bit 0 of the field indicates whether the page is a
tail page, and if set, the remaining bits represent a pointer to the
head page.

For cases when size of struct page is power-of-2, change the encoding of
compound_info to store a mask that can be applied to the virtual address
of the tail page in order to access the head page. It is possible
because struct page of the head page is naturally aligned with regards
to order of the page.

The significant impact of this modification is that all tail pages of
the same order will now have identical 'compound_info', regardless of
the compound page they are associated with. This paves the way for
eliminating fake heads.

The HugeTLB Vmemmap Optimization (HVO) creates fake heads and it is only
applied when the sizeof(struct page) is power-of-2. Having identical
tail pages allows the same page to be mapped into the vmemmap of all
pages, maintaining memory savings without fake heads.

If sizeof(struct page) is not power-of-2, there is no functional
changes.

Limit mask usage to HugeTLB vmemmap optimization (HVO) where it makes
a difference. The approach with mask would work in the wider set of
conditions, but it requires validating that struct pages are naturally
aligned for all orders up to the MAX_FOLIO_ORDER, which can be tricky.

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Zi Yan <ziy@nvidia.com>
---
 include/linux/page-flags.h | 81 ++++++++++++++++++++++++++++++++++----
 mm/slab.h                  | 16 ++++++--
 mm/util.c                  | 16 ++++++--
 3 files changed, 97 insertions(+), 16 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index d14a17ffb55b..8f2c7fbc739b 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -198,6 +198,29 @@ enum pageflags {
 
 #ifndef __GENERATING_BOUNDS_H
 
+/*
+ * For tail pages, if the size of struct page is power-of-2 ->compound_info
+ * encodes the mask that converts the address of the tail page address to
+ * the head page address.
+ *
+ * Otherwise, ->compound_info has direct pointer to head pages.
+ */
+static __always_inline bool compound_info_has_mask(void)
+{
+	/*
+	 * Limit mask usage to HugeTLB vmemmap optimization (HVO) where it
+	 * makes a difference.
+	 *
+	 * The approach with mask would work in the wider set of conditions,
+	 * but it requires validating that struct pages are naturally aligned
+	 * for all orders up to the MAX_FOLIO_ORDER, which can be tricky.
+	 */
+	if (!IS_ENABLED(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP))
+		return false;
+
+	return is_power_of_2(sizeof(struct page));
+}
+
 #ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
 DECLARE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key);
 
@@ -210,6 +233,10 @@ static __always_inline const struct page *page_fixed_fake_head(const struct page
 	if (!static_branch_unlikely(&hugetlb_optimize_vmemmap_key))
 		return page;
 
+	/* Fake heads only exists if compound_info_has_mask() is true */
+	if (!compound_info_has_mask())
+		return page;
+
 	/*
 	 * Only addresses aligned with PAGE_SIZE of struct page may be fake head
 	 * struct page. The alignment check aims to avoid access the fields (
@@ -223,10 +250,14 @@ static __always_inline const struct page *page_fixed_fake_head(const struct page
 		 * because the @page is a compound page composed with at least
 		 * two contiguous pages.
 		 */
-		unsigned long head = READ_ONCE(page[1].compound_info);
+		unsigned long info = READ_ONCE(page[1].compound_info);
 
-		if (likely(head & 1))
-			return (const struct page *)(head - 1);
+		/* See set_compound_head() */
+		if (likely(info & 1)) {
+			unsigned long p = (unsigned long)page;
+
+			return (const struct page *)(p & info);
+		}
 	}
 	return page;
 }
@@ -281,11 +312,26 @@ static __always_inline int page_is_fake_head(const struct page *page)
 
 static __always_inline unsigned long _compound_head(const struct page *page)
 {
-	unsigned long head = READ_ONCE(page->compound_info);
+	unsigned long info = READ_ONCE(page->compound_info);
 
-	if (unlikely(head & 1))
-		return head - 1;
-	return (unsigned long)page_fixed_fake_head(page);
+	/* Bit 0 encodes PageTail() */
+	if (!(info & 1))
+		return (unsigned long)page_fixed_fake_head(page);
+
+	/*
+	 * If compound_info_has_mask() is false, the rest of compound_info is
+	 * the pointer to the head page.
+	 */
+	if (!compound_info_has_mask())
+		return info - 1;
+
+	/*
+	 * If compoun_info_has_mask() is true the rest of the info encodes
+	 * the mask that converts the address of the tail page to the head page.
+	 *
+	 * No need to clear bit 0 in the mask as 'page' always has it clear.
+	 */
+	return (unsigned long)page & info;
 }
 
 #define compound_head(page)	((typeof(page))_compound_head(page))
@@ -294,7 +340,26 @@ static __always_inline void set_compound_head(struct page *page,
 					      const struct page *head,
 					      unsigned int order)
 {
-	WRITE_ONCE(page->compound_info, (unsigned long)head + 1);
+	unsigned int shift;
+	unsigned long mask;
+
+	if (!compound_info_has_mask()) {
+		WRITE_ONCE(page->compound_info, (unsigned long)head | 1);
+		return;
+	}
+
+	/*
+	 * If the size of struct page is power-of-2, bits [shift:0] of the
+	 * virtual address of compound head are zero.
+	 *
+	 * Calculate mask that can be applied to the virtual address of
+	 * the tail page to get address of the head page.
+	 */
+	shift = order + order_base_2(sizeof(struct page));
+	mask = GENMASK(BITS_PER_LONG - 1, shift);
+
+	/* Bit 0 encodes PageTail() */
+	WRITE_ONCE(page->compound_info, mask | 1);
 }
 
 static __always_inline void clear_compound_head(struct page *page)
diff --git a/mm/slab.h b/mm/slab.h
index 8a2a9c6c697b..f68c3ac8126f 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -137,11 +137,19 @@ static_assert(IS_ALIGNED(offsetof(struct slab, freelist), sizeof(struct freelist
  */
 static inline struct slab *page_slab(const struct page *page)
 {
-	unsigned long head;
+	unsigned long info;
+
+	info = READ_ONCE(page->compound_info);
+	if (info & 1) {
+		/* See compound_head() */
+		if (compound_info_has_mask()) {
+			unsigned long p = (unsigned long)page;
+			page = (struct page *)(p & info);
+		} else {
+			page = (struct page *)(info - 1);
+		}
+	}
 
-	head = READ_ONCE(page->compound_head);
-	if (head & 1)
-		page = (struct page *)(head - 1);
 	if (data_race(page->page_type >> 24) != PGTY_slab)
 		page = NULL;
 
diff --git a/mm/util.c b/mm/util.c
index 3ebcb9e6035c..20dccf2881d7 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1237,7 +1237,7 @@ static void set_ps_flags(struct page_snapshot *ps, const struct folio *folio,
  */
 void snapshot_page(struct page_snapshot *ps, const struct page *page)
 {
-	unsigned long head, nr_pages = 1;
+	unsigned long info, nr_pages = 1;
 	struct folio *foliop;
 	int loops = 5;
 
@@ -1247,8 +1247,8 @@ void snapshot_page(struct page_snapshot *ps, const struct page *page)
 again:
 	memset(&ps->folio_snapshot, 0, sizeof(struct folio));
 	memcpy(&ps->page_snapshot, page, sizeof(*page));
-	head = ps->page_snapshot.compound_info;
-	if ((head & 1) == 0) {
+	info = ps->page_snapshot.compound_info;
+	if (!(info & 1)) {
 		ps->idx = 0;
 		foliop = (struct folio *)&ps->page_snapshot;
 		if (!folio_test_large(foliop)) {
@@ -1259,7 +1259,15 @@ void snapshot_page(struct page_snapshot *ps, const struct page *page)
 		}
 		foliop = (struct folio *)page;
 	} else {
-		foliop = (struct folio *)(head - 1);
+		/* See compound_head() */
+		if (compound_info_has_mask()) {
+			unsigned long p = (unsigned long)page;
+
+			foliop = (struct folio *)(p & info);
+		} else {
+			foliop = (struct folio *)(info - 1);
+		}
+
 		ps->idx = folio_page_idx(foliop, page);
 	}
 
-- 
2.51.2

[PATCHv5 08/17] mm: Make page_zonenum() use head page

[Kiryl Shutsemau]

With the upcoming changes to HVO, a single page of tail struct pages
will be shared across all huge pages of the same order on a node. Since
huge pages on the same node may belong to different zones, the zone
information stored in shared tail page flags would be incorrect.

Always fetch zone information from the head page, which has unique and
correct zone flags for each compound page.

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Acked-by: Zi Yan <ziy@nvidia.com>
---
 include/linux/mmzone.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index be8ce40b5638..192143b5cdc0 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1219,6 +1219,7 @@ static inline enum zone_type memdesc_zonenum(memdesc_flags_t flags)
 
 static inline enum zone_type page_zonenum(const struct page *page)
 {
+	page = compound_head(page);
 	return memdesc_zonenum(page->flags);
 }
 
-- 
2.51.2

[PATCHv5 09/17] mm/sparse: Check memmap alignment for compound_info_has_mask()

[Kiryl Shutsemau]

If page->compound_info encodes a mask, it is expected that vmemmap to be
naturally aligned to the maximum folio size.

Trigger a BUG() for CONFIG_DEBUG_VM=y or WARN() otherwise.

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Acked-by: Zi Yan <ziy@nvidia.com>
---
 mm/sparse.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/mm/sparse.c b/mm/sparse.c
index b5b2b6f7041b..9c0f4015778c 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -600,6 +600,19 @@ void __init sparse_init(void)
 	BUILD_BUG_ON(!is_power_of_2(sizeof(struct mem_section)));
 	memblocks_present();
 
+	if (compound_info_has_mask()) {
+		unsigned long alignment;
+		bool aligned;
+
+		alignment = MAX_FOLIO_NR_PAGES * sizeof(struct page);
+		aligned = IS_ALIGNED((unsigned long) pfn_to_page(0), alignment);
+
+		if (IS_ENABLED(CONFIG_DEBUG_VM))
+			BUG_ON(!aligned);
+		else
+			WARN_ON(!aligned);
+	}
+
 	pnum_begin = first_present_section_nr();
 	nid_begin = sparse_early_nid(__nr_to_section(pnum_begin));
 
-- 
2.51.2

Re: [PATCHv5 09/17] mm/sparse: Check memmap alignment for compound_info_has_mask()

[Muchun Song]

> On Jan 28, 2026, at 21:54, Kiryl Shutsemau <kas@kernel.org> wrote:
> 
> If page->compound_info encodes a mask, it is expected that vmemmap to be
> naturally aligned to the maximum folio size.
> 
> Trigger a BUG() for CONFIG_DEBUG_VM=y or WARN() otherwise.
> 
> Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
> Acked-by: Zi Yan <ziy@nvidia.com>
> ---
> mm/sparse.c | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
> 
> diff --git a/mm/sparse.c b/mm/sparse.c
> index b5b2b6f7041b..9c0f4015778c 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -600,6 +600,19 @@ void __init sparse_init(void)
> BUILD_BUG_ON(!is_power_of_2(sizeof(struct mem_section)));
> 	memblocks_present();
> 
> + 	if (compound_info_has_mask()) {
> + 		unsigned long alignment;
> + 		bool aligned;
> +
> + 		alignment = MAX_FOLIO_NR_PAGES * sizeof(struct page);
> + 		aligned = IS_ALIGNED((unsigned long) pfn_to_page(0), alignment);
> +
> + 		if (IS_ENABLED(CONFIG_DEBUG_VM))
> + 			BUG_ON(!aligned);
> + 		else
> + 			WARN_ON(!aligned);

Since you’ve fixed all the problematic architectures, I don’t believe
we’ll ever hit the WARN or BUG here anymore.

I think we can now simplify the code further and just use VM_BUG_ON:
if any architecture changes in the future, the misalignment will be
caught during testing, so we won’t need to worry about it at run-time.

> + 	}
> +
> 	pnum_begin = first_present_section_nr();
> 	nid_begin = sparse_early_nid(__nr_to_section(pnum_begin));
> 
> -- 
> 2.51.2
> 

Re: [PATCHv5 09/17] mm/sparse: Check memmap alignment for compound_info_has_mask()

[Zi Yan]

On 28 Jan 2026, at 22:00, Muchun Song wrote:

>> On Jan 28, 2026, at 21:54, Kiryl Shutsemau <kas@kernel.org> wrote:
>>
>> If page->compound_info encodes a mask, it is expected that vmemmap to be
>> naturally aligned to the maximum folio size.
>>
>> Trigger a BUG() for CONFIG_DEBUG_VM=y or WARN() otherwise.
>>
>> Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
>> Acked-by: Zi Yan <ziy@nvidia.com>
>> ---
>> mm/sparse.c | 13 +++++++++++++
>> 1 file changed, 13 insertions(+)
>>
>> diff --git a/mm/sparse.c b/mm/sparse.c
>> index b5b2b6f7041b..9c0f4015778c 100644
>> --- a/mm/sparse.c
>> +++ b/mm/sparse.c
>> @@ -600,6 +600,19 @@ void __init sparse_init(void)
>> BUILD_BUG_ON(!is_power_of_2(sizeof(struct mem_section)));
>> 	memblocks_present();
>>
>> + 	if (compound_info_has_mask()) {
>> + 		unsigned long alignment;
>> + 		bool aligned;
>> +
>> + 		alignment = MAX_FOLIO_NR_PAGES * sizeof(struct page);
>> + 		aligned = IS_ALIGNED((unsigned long) pfn_to_page(0), alignment);
>> +
>> + 		if (IS_ENABLED(CONFIG_DEBUG_VM))
>> + 			BUG_ON(!aligned);
>> + 		else
>> + 			WARN_ON(!aligned);
>
> Since you’ve fixed all the problematic architectures, I don’t believe
> we’ll ever hit the WARN or BUG here anymore.
>
> I think we can now simplify the code further and just use VM_BUG_ON:
> if any architecture changes in the future, the misalignment will be
> caught during testing, so we won’t need to worry about it at run-time.
>

VM_WARN_ON should be sufficient, since bots should report warnings
from any patch/change.

>> + 	}
>> +
>> 	pnum_begin = first_present_section_nr();
>> 	nid_begin = sparse_early_nid(__nr_to_section(pnum_begin));
>>
>> -- 
>> 2.51.2
>>


Best Regards,
Yan, Zi

Re: [PATCHv5 09/17] mm/sparse: Check memmap alignment for compound_info_has_mask()

[Muchun Song]

> On Jan 29, 2026, at 11:10, Zi Yan <ziy@nvidia.com> wrote:
> 
> On 28 Jan 2026, at 22:00, Muchun Song wrote:
> 
>>> On Jan 28, 2026, at 21:54, Kiryl Shutsemau <kas@kernel.org> wrote:
>>> 
>>> If page->compound_info encodes a mask, it is expected that vmemmap to be
>>> naturally aligned to the maximum folio size.
>>> 
>>> Trigger a BUG() for CONFIG_DEBUG_VM=y or WARN() otherwise.
>>> 
>>> Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
>>> Acked-by: Zi Yan <ziy@nvidia.com>
>>> ---
>>> mm/sparse.c | 13 +++++++++++++
>>> 1 file changed, 13 insertions(+)
>>> 
>>> diff --git a/mm/sparse.c b/mm/sparse.c
>>> index b5b2b6f7041b..9c0f4015778c 100644
>>> --- a/mm/sparse.c
>>> +++ b/mm/sparse.c
>>> @@ -600,6 +600,19 @@ void __init sparse_init(void)
>>> BUILD_BUG_ON(!is_power_of_2(sizeof(struct mem_section)));
>>> memblocks_present();
>>> 
>>> +  if (compound_info_has_mask()) {
>>> +  	unsigned long alignment;
>>> +  	bool aligned;
>>> +
>>> +  	alignment = MAX_FOLIO_NR_PAGES * sizeof(struct page);
>>> +  	aligned = IS_ALIGNED((unsigned long) pfn_to_page(0), alignment);
>>> +
>>> +  	if (IS_ENABLED(CONFIG_DEBUG_VM))
>>> +  		BUG_ON(!aligned);
>>> +  	else
>>> +  		WARN_ON(!aligned);
>> 
>> Since you’ve fixed all the problematic architectures, I don’t believe
>> we’ll ever hit the WARN or BUG here anymore.
>> 
>> I think we can now simplify the code further and just use VM_BUG_ON:
>> if any architecture changes in the future, the misalignment will be
>> caught during testing, so we won’t need to worry about it at run-time.
>> 
> 
> VM_WARN_ON should be sufficient, since bots should report warnings
> from any patch/change.

I’m not sure a WARN will get developers’ attention, since the message
is unlikely to have any visible consequences and only fires on
allocations with a special order.

> 
>>> +  }
>>> +
>>> pnum_begin = first_present_section_nr();
>>> nid_begin = sparse_early_nid(__nr_to_section(pnum_begin));
>>> 
>>> -- 
>>> 2.51.2
>>> 
> 
> 
> Best Regards,
> Yan, Zi

Re: [PATCHv5 09/17] mm/sparse: Check memmap alignment for compound_info_has_mask()

[Zi Yan]

On 28 Jan 2026, at 22:23, Muchun Song wrote:

>> On Jan 29, 2026, at 11:10, Zi Yan <ziy@nvidia.com> wrote:
>>
>> On 28 Jan 2026, at 22:00, Muchun Song wrote:
>>
>>>> On Jan 28, 2026, at 21:54, Kiryl Shutsemau <kas@kernel.org> wrote:
>>>>
>>>> If page->compound_info encodes a mask, it is expected that vmemmap to be
>>>> naturally aligned to the maximum folio size.
>>>>
>>>> Trigger a BUG() for CONFIG_DEBUG_VM=y or WARN() otherwise.
>>>>
>>>> Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
>>>> Acked-by: Zi Yan <ziy@nvidia.com>
>>>> ---
>>>> mm/sparse.c | 13 +++++++++++++
>>>> 1 file changed, 13 insertions(+)
>>>>
>>>> diff --git a/mm/sparse.c b/mm/sparse.c
>>>> index b5b2b6f7041b..9c0f4015778c 100644
>>>> --- a/mm/sparse.c
>>>> +++ b/mm/sparse.c
>>>> @@ -600,6 +600,19 @@ void __init sparse_init(void)
>>>> BUILD_BUG_ON(!is_power_of_2(sizeof(struct mem_section)));
>>>> memblocks_present();
>>>>
>>>> +  if (compound_info_has_mask()) {
>>>> +  	unsigned long alignment;
>>>> +  	bool aligned;
>>>> +
>>>> +  	alignment = MAX_FOLIO_NR_PAGES * sizeof(struct page);
>>>> +  	aligned = IS_ALIGNED((unsigned long) pfn_to_page(0), alignment);
>>>> +
>>>> +  	if (IS_ENABLED(CONFIG_DEBUG_VM))
>>>> +  		BUG_ON(!aligned);
>>>> +  	else
>>>> +  		WARN_ON(!aligned);
>>>
>>> Since you’ve fixed all the problematic architectures, I don’t believe
>>> we’ll ever hit the WARN or BUG here anymore.
>>>
>>> I think we can now simplify the code further and just use VM_BUG_ON:
>>> if any architecture changes in the future, the misalignment will be
>>> caught during testing, so we won’t need to worry about it at run-time.
>>>
>>
>> VM_WARN_ON should be sufficient, since bots should report warnings
>> from any patch/change.
>
> I’m not sure a WARN will get developers’ attention, since the message
> is unlikely to have any visible consequences and only fires on
> allocations with a special order.

If a developer misses the WARN and the patch gets into linux-mm or linux-next,
kernel test robot runs selftests on the kernel and reports any warnings
to the mailing list. Do we have any related test in selftests/mm? That should
help us catch anything if a developer does not catch it.

Best Regards,
Yan, Zi

Re: [PATCHv5 09/17] mm/sparse: Check memmap alignment for compound_info_has_mask()

[Muchun Song]

> On Jan 29, 2026, at 11:29, Zi Yan <ziy@nvidia.com> wrote:
> 
> On 28 Jan 2026, at 22:23, Muchun Song wrote:
> 
>>> On Jan 29, 2026, at 11:10, Zi Yan <ziy@nvidia.com> wrote:
>>> 
>>> On 28 Jan 2026, at 22:00, Muchun Song wrote:
>>> 
>>>>> On Jan 28, 2026, at 21:54, Kiryl Shutsemau <kas@kernel.org> wrote:
>>>>> 
>>>>> If page->compound_info encodes a mask, it is expected that vmemmap to be
>>>>> naturally aligned to the maximum folio size.
>>>>> 
>>>>> Trigger a BUG() for CONFIG_DEBUG_VM=y or WARN() otherwise.
>>>>> 
>>>>> Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
>>>>> Acked-by: Zi Yan <ziy@nvidia.com>
>>>>> ---
>>>>> mm/sparse.c | 13 +++++++++++++
>>>>> 1 file changed, 13 insertions(+)
>>>>> 
>>>>> diff --git a/mm/sparse.c b/mm/sparse.c
>>>>> index b5b2b6f7041b..9c0f4015778c 100644
>>>>> --- a/mm/sparse.c
>>>>> +++ b/mm/sparse.c
>>>>> @@ -600,6 +600,19 @@ void __init sparse_init(void)
>>>>> BUILD_BUG_ON(!is_power_of_2(sizeof(struct mem_section)));
>>>>> memblocks_present();
>>>>> 
>>>>> +  if (compound_info_has_mask()) {
>>>>> +   unsigned long alignment;
>>>>> +   bool aligned;
>>>>> +
>>>>> +   alignment = MAX_FOLIO_NR_PAGES * sizeof(struct page);
>>>>> +   aligned = IS_ALIGNED((unsigned long) pfn_to_page(0), alignment);
>>>>> +
>>>>> +   if (IS_ENABLED(CONFIG_DEBUG_VM))
>>>>> +   BUG_ON(!aligned);
>>>>> +   else
>>>>> +   WARN_ON(!aligned);
>>>> 
>>>> Since you’ve fixed all the problematic architectures, I don’t believe
>>>> we’ll ever hit the WARN or BUG here anymore.
>>>> 
>>>> I think we can now simplify the code further and just use VM_BUG_ON:
>>>> if any architecture changes in the future, the misalignment will be
>>>> caught during testing, so we won’t need to worry about it at run-time.
>>>> 
>>> 
>>> VM_WARN_ON should be sufficient, since bots should report warnings
>>> from any patch/change.
>> 
>> I’m not sure a WARN will get developers’ attention, since the message
>> is unlikely to have any visible consequences and only fires on
>> allocations with a special order.
> 
> If a developer misses the WARN and the patch gets into linux-mm or linux-next,
> kernel test robot runs selftests on the kernel and reports any warnings
> to the mailing list. Do we have any related test in selftests/mm? That should
> help us catch anything if a developer does not catch it.

I looked at the selftest and it doesn’t seem to have a test that
allocates at MAX_FOLIO_ORDER and checks that it works correctly.

> 
> Best Regards,
> Yan, Zi

Re: [PATCHv5 09/17] mm/sparse: Check memmap alignment for compound_info_has_mask()

[Zi Yan]

On 29 Jan 2026, at 2:03, Muchun Song wrote:

>> On Jan 29, 2026, at 11:29, Zi Yan <ziy@nvidia.com> wrote:
>>
>> On 28 Jan 2026, at 22:23, Muchun Song wrote:
>>
>>>> On Jan 29, 2026, at 11:10, Zi Yan <ziy@nvidia.com> wrote:
>>>>
>>>> On 28 Jan 2026, at 22:00, Muchun Song wrote:
>>>>
>>>>>> On Jan 28, 2026, at 21:54, Kiryl Shutsemau <kas@kernel.org> wrote:
>>>>>>
>>>>>> If page->compound_info encodes a mask, it is expected that vmemmap to be
>>>>>> naturally aligned to the maximum folio size.
>>>>>>
>>>>>> Trigger a BUG() for CONFIG_DEBUG_VM=y or WARN() otherwise.
>>>>>>
>>>>>> Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
>>>>>> Acked-by: Zi Yan <ziy@nvidia.com>
>>>>>> ---
>>>>>> mm/sparse.c | 13 +++++++++++++
>>>>>> 1 file changed, 13 insertions(+)
>>>>>>
>>>>>> diff --git a/mm/sparse.c b/mm/sparse.c
>>>>>> index b5b2b6f7041b..9c0f4015778c 100644
>>>>>> --- a/mm/sparse.c
>>>>>> +++ b/mm/sparse.c
>>>>>> @@ -600,6 +600,19 @@ void __init sparse_init(void)
>>>>>> BUILD_BUG_ON(!is_power_of_2(sizeof(struct mem_section)));
>>>>>> memblocks_present();
>>>>>>
>>>>>> +  if (compound_info_has_mask()) {
>>>>>> +   unsigned long alignment;
>>>>>> +   bool aligned;
>>>>>> +
>>>>>> +   alignment = MAX_FOLIO_NR_PAGES * sizeof(struct page);
>>>>>> +   aligned = IS_ALIGNED((unsigned long) pfn_to_page(0), alignment);
>>>>>> +
>>>>>> +   if (IS_ENABLED(CONFIG_DEBUG_VM))
>>>>>> +   BUG_ON(!aligned);
>>>>>> +   else
>>>>>> +   WARN_ON(!aligned);
>>>>>
>>>>> Since you’ve fixed all the problematic architectures, I don’t believe
>>>>> we’ll ever hit the WARN or BUG here anymore.
>>>>>
>>>>> I think we can now simplify the code further and just use VM_BUG_ON:
>>>>> if any architecture changes in the future, the misalignment will be
>>>>> caught during testing, so we won’t need to worry about it at run-time.
>>>>>
>>>>
>>>> VM_WARN_ON should be sufficient, since bots should report warnings
>>>> from any patch/change.
>>>
>>> I’m not sure a WARN will get developers’ attention, since the message
>>> is unlikely to have any visible consequences and only fires on
>>> allocations with a special order.
>>
>> If a developer misses the WARN and the patch gets into linux-mm or linux-next,
>> kernel test robot runs selftests on the kernel and reports any warnings
>> to the mailing list. Do we have any related test in selftests/mm? That should
>> help us catch anything if a developer does not catch it.
>
> I looked at the selftest and it doesn’t seem to have a test that
> allocates at MAX_FOLIO_ORDER and checks that it works correctly.

OK, we probably need a selftest for it. In terms of using VM_BUG_ON or
VM_WARN_ON, I leave that decision to you and Kiryl.

Thank you for the clarification.

Best Regards,
Yan, Zi

[PATCHv5 10/17] mm/hugetlb: Refactor code around vmemmap_walk

[Kiryl Shutsemau]

To prepare for removing fake head pages, the vmemmap_walk code is being
reworked.

The reuse_page and reuse_addr variables are being eliminated. There will
no longer be an expectation regarding the reuse address in relation to
the operated range. Instead, the caller will provide head and tail
vmemmap pages.

Currently, vmemmap_head and vmemmap_tail are set to the same page, but
this will change in the future.

The only functional change is that __hugetlb_vmemmap_optimize_folio()
will abandon optimization if memory allocation fails.

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
---
 mm/hugetlb_vmemmap.c | 226 +++++++++++++++++--------------------------
 1 file changed, 90 insertions(+), 136 deletions(-)

diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index a9280259e12a..a39a301e08b9 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -25,8 +25,8 @@
  *
  * @remap_pte:		called for each lowest-level entry (PTE).
  * @nr_walked:		the number of walked pte.
- * @reuse_page:		the page which is reused for the tail vmemmap pages.
- * @reuse_addr:		the virtual address of the @reuse_page page.
+ * @vmemmap_head:	the page to be installed as first in the vmemmap range
+ * @vmemmap_tail:	the page to be installed as non-first in the vmemmap range
  * @vmemmap_pages:	the list head of the vmemmap pages that can be freed
  *			or is mapped from.
  * @flags:		used to modify behavior in vmemmap page table walking
@@ -35,11 +35,13 @@
 struct vmemmap_remap_walk {
 	void			(*remap_pte)(pte_t *pte, unsigned long addr,
 					     struct vmemmap_remap_walk *walk);
+
 	unsigned long		nr_walked;
-	struct page		*reuse_page;
-	unsigned long		reuse_addr;
+	struct page		*vmemmap_head;
+	struct page		*vmemmap_tail;
 	struct list_head	*vmemmap_pages;
 
+
 /* Skip the TLB flush when we split the PMD */
 #define VMEMMAP_SPLIT_NO_TLB_FLUSH	BIT(0)
 /* Skip the TLB flush when we remap the PTE */
@@ -141,14 +143,7 @@ static int vmemmap_pte_entry(pte_t *pte, unsigned long addr,
 {
 	struct vmemmap_remap_walk *vmemmap_walk = walk->private;
 
-	/*
-	 * The reuse_page is found 'first' in page table walking before
-	 * starting remapping.
-	 */
-	if (!vmemmap_walk->reuse_page)
-		vmemmap_walk->reuse_page = pte_page(ptep_get(pte));
-	else
-		vmemmap_walk->remap_pte(pte, addr, vmemmap_walk);
+	vmemmap_walk->remap_pte(pte, addr, vmemmap_walk);
 	vmemmap_walk->nr_walked++;
 
 	return 0;
@@ -208,18 +203,12 @@ static void free_vmemmap_page_list(struct list_head *list)
 static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
 			      struct vmemmap_remap_walk *walk)
 {
-	/*
-	 * Remap the tail pages as read-only to catch illegal write operation
-	 * to the tail pages.
-	 */
-	pgprot_t pgprot = PAGE_KERNEL_RO;
 	struct page *page = pte_page(ptep_get(pte));
 	pte_t entry;
 
 	/* Remapping the head page requires r/w */
-	if (unlikely(addr == walk->reuse_addr)) {
-		pgprot = PAGE_KERNEL;
-		list_del(&walk->reuse_page->lru);
+	if (unlikely(walk->nr_walked == 0 && walk->vmemmap_head)) {
+		list_del(&walk->vmemmap_head->lru);
 
 		/*
 		 * Makes sure that preceding stores to the page contents from
@@ -227,53 +216,50 @@ static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
 		 * write.
 		 */
 		smp_wmb();
+
+		entry = mk_pte(walk->vmemmap_head, PAGE_KERNEL);
+	} else {
+		/*
+		 * Remap the tail pages as read-only to catch illegal write
+		 * operation to the tail pages.
+		 */
+		entry = mk_pte(walk->vmemmap_tail, PAGE_KERNEL_RO);
 	}
 
-	entry = mk_pte(walk->reuse_page, pgprot);
 	list_add(&page->lru, walk->vmemmap_pages);
 	set_pte_at(&init_mm, addr, pte, entry);
 }
 
-/*
- * How many struct page structs need to be reset. When we reuse the head
- * struct page, the special metadata (e.g. page->flags or page->mapping)
- * cannot copy to the tail struct page structs. The invalid value will be
- * checked in the free_tail_page_prepare(). In order to avoid the message
- * of "corrupted mapping in tail page". We need to reset at least 4 (one
- * head struct page struct and three tail struct page structs) struct page
- * structs.
- */
-#define NR_RESET_STRUCT_PAGE		4
-
-static inline void reset_struct_pages(struct page *start)
-{
-	struct page *from = start + NR_RESET_STRUCT_PAGE;
-
-	BUILD_BUG_ON(NR_RESET_STRUCT_PAGE * 2 > PAGE_SIZE / sizeof(struct page));
-	memcpy(start, from, sizeof(*from) * NR_RESET_STRUCT_PAGE);
-}
-
 static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
 				struct vmemmap_remap_walk *walk)
 {
-	pgprot_t pgprot = PAGE_KERNEL;
 	struct page *page;
-	void *to;
-
-	BUG_ON(pte_page(ptep_get(pte)) != walk->reuse_page);
+	struct page *from, *to;
 
 	page = list_first_entry(walk->vmemmap_pages, struct page, lru);
 	list_del(&page->lru);
+
+	/*
+	 * Initialize tail pages in the newly allocated vmemmap page.
+	 *
+	 * There is folio-scope metadata that is encoded in the first few
+	 * tail pages.
+	 *
+	 * Use the value last tail page in the page with the head page
+	 * to initialize the rest of tail pages.
+	 */
+	from = compound_head((struct page *)addr) +
+		PAGE_SIZE / sizeof(struct page) - 1;
 	to = page_to_virt(page);
-	copy_page(to, (void *)walk->reuse_addr);
-	reset_struct_pages(to);
+	for (int i = 0; i < PAGE_SIZE / sizeof(struct page); i++, to++)
+		*to = *from;
 
 	/*
 	 * Makes sure that preceding stores to the page contents become visible
 	 * before the set_pte_at() write.
 	 */
 	smp_wmb();
-	set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
+	set_pte_at(&init_mm, addr, pte, mk_pte(page, PAGE_KERNEL));
 }
 
 /**
@@ -283,33 +269,28 @@ static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
  *             to remap.
  * @end:       end address of the vmemmap virtual address range that we want to
  *             remap.
- * @reuse:     reuse address.
- *
  * Return: %0 on success, negative error code otherwise.
  */
-static int vmemmap_remap_split(unsigned long start, unsigned long end,
-			       unsigned long reuse)
+static int vmemmap_remap_split(unsigned long start, unsigned long end)
 {
 	struct vmemmap_remap_walk walk = {
 		.remap_pte	= NULL,
 		.flags		= VMEMMAP_SPLIT_NO_TLB_FLUSH,
 	};
 
-	/* See the comment in the vmemmap_remap_free(). */
-	BUG_ON(start - reuse != PAGE_SIZE);
-
-	return vmemmap_remap_range(reuse, end, &walk);
+	return vmemmap_remap_range(start, end, &walk);
 }
 
 /**
  * vmemmap_remap_free - remap the vmemmap virtual address range [@start, @end)
- *			to the page which @reuse is mapped to, then free vmemmap
- *			which the range are mapped to.
+ *			to use @vmemmap_head/tail, then free vmemmap which
+ *			the range are mapped to.
  * @start:	start address of the vmemmap virtual address range that we want
  *		to remap.
  * @end:	end address of the vmemmap virtual address range that we want to
  *		remap.
- * @reuse:	reuse address.
+ * @vmemmap_head: the page to be installed as first in the vmemmap range
+ * @vmemmap_tail: the page to be installed as non-first in the vmemmap range
  * @vmemmap_pages: list to deposit vmemmap pages to be freed.  It is callers
  *		responsibility to free pages.
  * @flags:	modifications to vmemmap_remap_walk flags
@@ -317,69 +298,38 @@ static int vmemmap_remap_split(unsigned long start, unsigned long end,
  * Return: %0 on success, negative error code otherwise.
  */
 static int vmemmap_remap_free(unsigned long start, unsigned long end,
-			      unsigned long reuse,
+			      struct page *vmemmap_head,
+			      struct page *vmemmap_tail,
 			      struct list_head *vmemmap_pages,
 			      unsigned long flags)
 {
 	int ret;
 	struct vmemmap_remap_walk walk = {
 		.remap_pte	= vmemmap_remap_pte,
-		.reuse_addr	= reuse,
+		.vmemmap_head	= vmemmap_head,
+		.vmemmap_tail	= vmemmap_tail,
 		.vmemmap_pages	= vmemmap_pages,
 		.flags		= flags,
 	};
-	int nid = page_to_nid((struct page *)reuse);
-	gfp_t gfp_mask = GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN;
+
+	ret = vmemmap_remap_range(start, end, &walk);
+	if (!ret || !walk.nr_walked)
+		return ret;
+
+	end = start + walk.nr_walked * PAGE_SIZE;
 
 	/*
-	 * Allocate a new head vmemmap page to avoid breaking a contiguous
-	 * block of struct page memory when freeing it back to page allocator
-	 * in free_vmemmap_page_list(). This will allow the likely contiguous
-	 * struct page backing memory to be kept contiguous and allowing for
-	 * more allocations of hugepages. Fallback to the currently
-	 * mapped head page in case should it fail to allocate.
+	 * vmemmap_pages contains pages from the previous vmemmap_remap_range()
+	 * call which failed.  These are pages which were removed from
+	 * the vmemmap. They will be restored in the following call.
 	 */
-	walk.reuse_page = alloc_pages_node(nid, gfp_mask, 0);
-	if (walk.reuse_page) {
-		copy_page(page_to_virt(walk.reuse_page),
-			  (void *)walk.reuse_addr);
-		list_add(&walk.reuse_page->lru, vmemmap_pages);
-		memmap_pages_add(1);
-	}
+	walk = (struct vmemmap_remap_walk) {
+		.remap_pte	= vmemmap_restore_pte,
+		.vmemmap_pages	= vmemmap_pages,
+		.flags		= 0,
+	};
 
-	/*
-	 * In order to make remapping routine most efficient for the huge pages,
-	 * the routine of vmemmap page table walking has the following rules
-	 * (see more details from the vmemmap_pte_range()):
-	 *
-	 * - The range [@start, @end) and the range [@reuse, @reuse + PAGE_SIZE)
-	 *   should be continuous.
-	 * - The @reuse address is part of the range [@reuse, @end) that we are
-	 *   walking which is passed to vmemmap_remap_range().
-	 * - The @reuse address is the first in the complete range.
-	 *
-	 * So we need to make sure that @start and @reuse meet the above rules.
-	 */
-	BUG_ON(start - reuse != PAGE_SIZE);
-
-	ret = vmemmap_remap_range(reuse, end, &walk);
-	if (ret && walk.nr_walked) {
-		end = reuse + walk.nr_walked * PAGE_SIZE;
-		/*
-		 * vmemmap_pages contains pages from the previous
-		 * vmemmap_remap_range call which failed.  These
-		 * are pages which were removed from the vmemmap.
-		 * They will be restored in the following call.
-		 */
-		walk = (struct vmemmap_remap_walk) {
-			.remap_pte	= vmemmap_restore_pte,
-			.reuse_addr	= reuse,
-			.vmemmap_pages	= vmemmap_pages,
-			.flags		= 0,
-		};
-
-		vmemmap_remap_range(reuse, end, &walk);
-	}
+	vmemmap_remap_range(start, end, &walk);
 
 	return ret;
 }
@@ -416,29 +366,24 @@ static int alloc_vmemmap_page_list(unsigned long start, unsigned long end,
  *		to remap.
  * @end:	end address of the vmemmap virtual address range that we want to
  *		remap.
- * @reuse:	reuse address.
  * @flags:	modifications to vmemmap_remap_walk flags
  *
  * Return: %0 on success, negative error code otherwise.
  */
 static int vmemmap_remap_alloc(unsigned long start, unsigned long end,
-			       unsigned long reuse, unsigned long flags)
+			       unsigned long flags)
 {
 	LIST_HEAD(vmemmap_pages);
 	struct vmemmap_remap_walk walk = {
 		.remap_pte	= vmemmap_restore_pte,
-		.reuse_addr	= reuse,
 		.vmemmap_pages	= &vmemmap_pages,
 		.flags		= flags,
 	};
 
-	/* See the comment in the vmemmap_remap_free(). */
-	BUG_ON(start - reuse != PAGE_SIZE);
-
 	if (alloc_vmemmap_page_list(start, end, &vmemmap_pages))
 		return -ENOMEM;
 
-	return vmemmap_remap_range(reuse, end, &walk);
+	return vmemmap_remap_range(start, end, &walk);
 }
 
 DEFINE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key);
@@ -455,8 +400,7 @@ static int __hugetlb_vmemmap_restore_folio(const struct hstate *h,
 					   struct folio *folio, unsigned long flags)
 {
 	int ret;
-	unsigned long vmemmap_start = (unsigned long)&folio->page, vmemmap_end;
-	unsigned long vmemmap_reuse;
+	unsigned long vmemmap_start, vmemmap_end;
 
 	VM_WARN_ON_ONCE_FOLIO(!folio_test_hugetlb(folio), folio);
 	VM_WARN_ON_ONCE_FOLIO(folio_ref_count(folio), folio);
@@ -467,18 +411,18 @@ static int __hugetlb_vmemmap_restore_folio(const struct hstate *h,
 	if (flags & VMEMMAP_SYNCHRONIZE_RCU)
 		synchronize_rcu();
 
+	vmemmap_start	= (unsigned long)&folio->page;
 	vmemmap_end	= vmemmap_start + hugetlb_vmemmap_size(h);
-	vmemmap_reuse	= vmemmap_start;
+
 	vmemmap_start	+= HUGETLB_VMEMMAP_RESERVE_SIZE;
 
 	/*
 	 * The pages which the vmemmap virtual address range [@vmemmap_start,
-	 * @vmemmap_end) are mapped to are freed to the buddy allocator, and
-	 * the range is mapped to the page which @vmemmap_reuse is mapped to.
+	 * @vmemmap_end) are mapped to are freed to the buddy allocator.
 	 * When a HugeTLB page is freed to the buddy allocator, previously
 	 * discarded vmemmap pages must be allocated and remapping.
 	 */
-	ret = vmemmap_remap_alloc(vmemmap_start, vmemmap_end, vmemmap_reuse, flags);
+	ret = vmemmap_remap_alloc(vmemmap_start, vmemmap_end, flags);
 	if (!ret) {
 		folio_clear_hugetlb_vmemmap_optimized(folio);
 		static_branch_dec(&hugetlb_optimize_vmemmap_key);
@@ -566,9 +510,9 @@ static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
 					    struct list_head *vmemmap_pages,
 					    unsigned long flags)
 {
-	int ret = 0;
-	unsigned long vmemmap_start = (unsigned long)&folio->page, vmemmap_end;
-	unsigned long vmemmap_reuse;
+	unsigned long vmemmap_start, vmemmap_end;
+	struct page *vmemmap_head, *vmemmap_tail;
+	int nid, ret = 0;
 
 	VM_WARN_ON_ONCE_FOLIO(!folio_test_hugetlb(folio), folio);
 	VM_WARN_ON_ONCE_FOLIO(folio_ref_count(folio), folio);
@@ -593,18 +537,30 @@ static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
 	 */
 	folio_set_hugetlb_vmemmap_optimized(folio);
 
+	nid = folio_nid(folio);
+	vmemmap_head = alloc_pages_node(nid, GFP_KERNEL, 0);
+	if (!vmemmap_head) {
+		ret = -ENOMEM;
+		goto out;
+	}
+
+	copy_page(page_to_virt(vmemmap_head), folio);
+	list_add(&vmemmap_head->lru, vmemmap_pages);
+	memmap_pages_add(1);
+
+	vmemmap_tail	= vmemmap_head;
+	vmemmap_start	= (unsigned long)&folio->page;
 	vmemmap_end	= vmemmap_start + hugetlb_vmemmap_size(h);
-	vmemmap_reuse	= vmemmap_start;
-	vmemmap_start	+= HUGETLB_VMEMMAP_RESERVE_SIZE;
 
 	/*
-	 * Remap the vmemmap virtual address range [@vmemmap_start, @vmemmap_end)
-	 * to the page which @vmemmap_reuse is mapped to.  Add pages previously
-	 * mapping the range to vmemmap_pages list so that they can be freed by
-	 * the caller.
+	 * Remap the vmemmap virtual address range [@vmemmap_start, @vmemmap_end).
+	 * Add pages previously mapping the range to vmemmap_pages list so that
+	 * they can be freed by the caller.
 	 */
-	ret = vmemmap_remap_free(vmemmap_start, vmemmap_end, vmemmap_reuse,
+	ret = vmemmap_remap_free(vmemmap_start, vmemmap_end,
+				 vmemmap_head, vmemmap_tail,
 				 vmemmap_pages, flags);
+out:
 	if (ret) {
 		static_branch_dec(&hugetlb_optimize_vmemmap_key);
 		folio_clear_hugetlb_vmemmap_optimized(folio);
@@ -633,21 +589,19 @@ void hugetlb_vmemmap_optimize_folio(const struct hstate *h, struct folio *folio)
 
 static int hugetlb_vmemmap_split_folio(const struct hstate *h, struct folio *folio)
 {
-	unsigned long vmemmap_start = (unsigned long)&folio->page, vmemmap_end;
-	unsigned long vmemmap_reuse;
+	unsigned long vmemmap_start, vmemmap_end;
 
 	if (!vmemmap_should_optimize_folio(h, folio))
 		return 0;
 
+	vmemmap_start	= (unsigned long)&folio->page;
 	vmemmap_end	= vmemmap_start + hugetlb_vmemmap_size(h);
-	vmemmap_reuse	= vmemmap_start;
-	vmemmap_start	+= HUGETLB_VMEMMAP_RESERVE_SIZE;
 
 	/*
 	 * Split PMDs on the vmemmap virtual address range [@vmemmap_start,
 	 * @vmemmap_end]
 	 */
-	return vmemmap_remap_split(vmemmap_start, vmemmap_end, vmemmap_reuse);
+	return vmemmap_remap_split(vmemmap_start, vmemmap_end);
 }
 
 static void __hugetlb_vmemmap_optimize_folios(struct hstate *h,
-- 
2.51.2

Re: [PATCHv5 10/17] mm/hugetlb: Refactor code around vmemmap_walk

[Muchun Song]

> On Jan 28, 2026, at 21:54, Kiryl Shutsemau <kas@kernel.org> wrote:
> 
> To prepare for removing fake head pages, the vmemmap_walk code is being
> reworked.
> 
> The reuse_page and reuse_addr variables are being eliminated. There will
> no longer be an expectation regarding the reuse address in relation to
> the operated range. Instead, the caller will provide head and tail
> vmemmap pages.
> 
> Currently, vmemmap_head and vmemmap_tail are set to the same page, but
> this will change in the future.
> 
> The only functional change is that __hugetlb_vmemmap_optimize_folio()
> will abandon optimization if memory allocation fails.
> 
> Signed-off-by: Kiryl Shutsemau <kas@kernel.org>

Reviewed-by: Muchun Song <muchun.song@linux.dev>

Thanks.

[PATCHv5 11/17] mm/hugetlb: Remove fake head pages

[Kiryl Shutsemau]

HugeTLB Vmemmap Optimization (HVO) reduces memory usage by freeing most
vmemmap pages for huge pages and remapping the freed range to a single
page containing the struct page metadata.

With the new mask-based compound_info encoding (for power-of-2 struct
page sizes), all tail pages of the same order are now identical
regardless of which compound page they belong to. This means the tail
pages can be truly shared without fake heads.

Allocate a single page of initialized tail struct pages per NUMA node
per order in the vmemmap_tails[] array in pglist_data. All huge pages of
that order on the node share this tail page, mapped read-only into their
vmemmap. The head page remains unique per huge page.

Redefine MAX_FOLIO_ORDER using ilog2(). The define has to produce a
compile-constant as it is used to specify vmemmap_tail array size.
For some reason, compiler is not able to solve get_order() at
compile-time, but ilog2() works.

Avoid PUD_ORDER to define MAX_FOLIO_ORDER as it adds dependency to
<linux/pgtable.h> which generates hard-to-break include loop.

This eliminates fake heads while maintaining the same memory savings,
and simplifies compound_head() by removing fake head detection.

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
---
 include/linux/mmzone.h | 18 +++++++++++++++--
 mm/hugetlb_vmemmap.c   | 36 ++++++++++++++++++++++++++++++++--
 mm/sparse-vmemmap.c    | 44 ++++++++++++++++++++++++++++++++++--------
 3 files changed, 86 insertions(+), 12 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 192143b5cdc0..698091c74dbb 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -81,13 +81,17 @@
  * currently expect (see CONFIG_HAVE_GIGANTIC_FOLIOS): with hugetlb, we expect
  * no folios larger than 16 GiB on 64bit and 1 GiB on 32bit.
  */
-#define MAX_FOLIO_ORDER		get_order(IS_ENABLED(CONFIG_64BIT) ? SZ_16G : SZ_1G)
+#ifdef CONFIG_64BIT
+#define MAX_FOLIO_ORDER		(ilog2(SZ_16G) - PAGE_SHIFT)
+#else
+#define MAX_FOLIO_ORDER		(ilog2(SZ_1G) - PAGE_SHIFT)
+#endif
 #else
 /*
  * Without hugetlb, gigantic folios that are bigger than a single PUD are
  * currently impossible.
  */
-#define MAX_FOLIO_ORDER		PUD_ORDER
+#define MAX_FOLIO_ORDER		(PUD_SHIFT - PAGE_SHIFT)
 #endif
 
 #define MAX_FOLIO_NR_PAGES	(1UL << MAX_FOLIO_ORDER)
@@ -1402,6 +1406,13 @@ struct memory_failure_stats {
 };
 #endif
 
+/*
+ * vmemmap optimization (like HVO) is only possible for page orders that fill
+ * two or more pages with struct pages.
+ */
+#define VMEMMAP_TAIL_MIN_ORDER (ilog2(2 * PAGE_SIZE / sizeof(struct page)))
+#define NR_VMEMMAP_TAILS (MAX_FOLIO_ORDER - VMEMMAP_TAIL_MIN_ORDER + 1)
+
 /*
  * On NUMA machines, each NUMA node would have a pg_data_t to describe
  * it's memory layout. On UMA machines there is a single pglist_data which
@@ -1550,6 +1561,9 @@ typedef struct pglist_data {
 #ifdef CONFIG_MEMORY_FAILURE
 	struct memory_failure_stats mf_stats;
 #endif
+#ifdef CONFIG_SPARSEMEM_VMEMMAP
+	unsigned long vmemmap_tails[NR_VMEMMAP_TAILS];
+#endif
 } pg_data_t;
 
 #define node_present_pages(nid)	(NODE_DATA(nid)->node_present_pages)
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index a39a301e08b9..f5f42b92dd7d 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -19,6 +19,7 @@
 
 #include <asm/tlbflush.h>
 #include "hugetlb_vmemmap.h"
+#include "internal.h"
 
 /**
  * struct vmemmap_remap_walk - walk vmemmap page table
@@ -505,6 +506,34 @@ static bool vmemmap_should_optimize_folio(const struct hstate *h, struct folio *
 	return true;
 }
 
+static struct page *vmemmap_get_tail(unsigned int order, int node)
+{
+	unsigned long pfn;
+	unsigned int idx;
+	struct page *tail, *p;
+
+	idx = order - VMEMMAP_TAIL_MIN_ORDER;
+	pfn = READ_ONCE(NODE_DATA(node)->vmemmap_tails[idx]);
+	if (pfn)
+		return pfn_to_page(pfn);
+
+	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++)
+		prep_compound_tail(p + i, NULL, order);
+
+	pfn = PHYS_PFN(virt_to_phys(p));
+	if (cmpxchg(&NODE_DATA(node)->vmemmap_tails[idx], 0, pfn)) {
+		__free_page(tail);
+		pfn = READ_ONCE(NODE_DATA(node)->vmemmap_tails[idx]);
+	}
+
+	return pfn_to_page(pfn);
+}
+
 static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
 					    struct folio *folio,
 					    struct list_head *vmemmap_pages,
@@ -520,6 +549,11 @@ static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
 	if (!vmemmap_should_optimize_folio(h, folio))
 		return ret;
 
+	nid = folio_nid(folio);
+	vmemmap_tail = vmemmap_get_tail(h->order, nid);
+	if (!vmemmap_tail)
+		return -ENOMEM;
+
 	static_branch_inc(&hugetlb_optimize_vmemmap_key);
 
 	if (flags & VMEMMAP_SYNCHRONIZE_RCU)
@@ -537,7 +571,6 @@ static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
 	 */
 	folio_set_hugetlb_vmemmap_optimized(folio);
 
-	nid = folio_nid(folio);
 	vmemmap_head = alloc_pages_node(nid, GFP_KERNEL, 0);
 	if (!vmemmap_head) {
 		ret = -ENOMEM;
@@ -548,7 +581,6 @@ static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
 	list_add(&vmemmap_head->lru, vmemmap_pages);
 	memmap_pages_add(1);
 
-	vmemmap_tail	= vmemmap_head;
 	vmemmap_start	= (unsigned long)&folio->page;
 	vmemmap_end	= vmemmap_start + hugetlb_vmemmap_size(h);
 
diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index 37522d6cb398..23abd06f1a4e 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -378,16 +378,45 @@ void vmemmap_wrprotect_hvo(unsigned long addr, unsigned long end,
 	}
 }
 
-/*
- * Populate vmemmap pages HVO-style. The first page contains the head
- * page and needed tail pages, the other ones are mirrors of the first
- * page.
- */
+static __meminit unsigned long vmemmap_get_tail(unsigned int order, int node)
+{
+	unsigned long pfn;
+	unsigned int idx;
+	struct page *p;
+
+	BUG_ON(order < VMEMMAP_TAIL_MIN_ORDER);
+	BUG_ON(order > MAX_FOLIO_ORDER);
+
+	idx = order - VMEMMAP_TAIL_MIN_ORDER;
+	pfn =  NODE_DATA(node)->vmemmap_tails[idx];
+	if (pfn)
+		return pfn;
+
+	p = vmemmap_alloc_block_zero(PAGE_SIZE, node);
+	if (!p)
+		return 0;
+
+	for (int i = 0; i < PAGE_SIZE / sizeof(struct page); i++)
+		prep_compound_tail(p + i, NULL, order);
+
+	pfn = PHYS_PFN(virt_to_phys(p));
+	NODE_DATA(node)->vmemmap_tails[idx] = pfn;
+
+	return pfn;
+}
+
 int __meminit vmemmap_populate_hvo(unsigned long addr, unsigned long end,
 				       int node, unsigned long headsize)
 {
+	unsigned long maddr, len, tail_pfn;
+	unsigned int order;
 	pte_t *pte;
-	unsigned long maddr;
+
+	len = end - addr;
+	order = ilog2(len * sizeof(struct page) / PAGE_SIZE);
+	tail_pfn = vmemmap_get_tail(order, node);
+	if (!tail_pfn)
+		return -ENOMEM;
 
 	for (maddr = addr; maddr < addr + headsize; maddr += PAGE_SIZE) {
 		pte = vmemmap_populate_address(maddr, node, NULL, -1, 0);
@@ -398,8 +427,7 @@ int __meminit vmemmap_populate_hvo(unsigned long addr, unsigned long end,
 	/*
 	 * Reuse the last page struct page mapped above for the rest.
 	 */
-	return vmemmap_populate_range(maddr, end, node, NULL,
-					pte_pfn(ptep_get(pte)), 0);
+	return vmemmap_populate_range(maddr, end, node, NULL, tail_pfn, 0);
 }
 
 void __weak __meminit vmemmap_set_pmd(pmd_t *pmd, void *p, int node,
-- 
2.51.2

Re: [PATCHv5 11/17] mm/hugetlb: Remove fake head pages

[Muchun Song]

On 2026/1/28 21:54, Kiryl Shutsemau wrote:
> HugeTLB Vmemmap Optimization (HVO) reduces memory usage by freeing most
> vmemmap pages for huge pages and remapping the freed range to a single
> page containing the struct page metadata.
>
> With the new mask-based compound_info encoding (for power-of-2 struct
> page sizes), all tail pages of the same order are now identical
> regardless of which compound page they belong to. This means the tail
> pages can be truly shared without fake heads.
>
> Allocate a single page of initialized tail struct pages per NUMA node
> per order in the vmemmap_tails[] array in pglist_data. All huge pages of
> that order on the node share this tail page, mapped read-only into their
> vmemmap. The head page remains unique per huge page.
>
> Redefine MAX_FOLIO_ORDER using ilog2(). The define has to produce a
> compile-constant as it is used to specify vmemmap_tail array size.
> For some reason, compiler is not able to solve get_order() at
> compile-time, but ilog2() works.
>
> Avoid PUD_ORDER to define MAX_FOLIO_ORDER as it adds dependency to
> <linux/pgtable.h> which generates hard-to-break include loop.
>
> This eliminates fake heads while maintaining the same memory savings,
> and simplifies compound_head() by removing fake head detection.
>
> Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
> ---
>   include/linux/mmzone.h | 18 +++++++++++++++--
>   mm/hugetlb_vmemmap.c   | 36 ++++++++++++++++++++++++++++++++--
>   mm/sparse-vmemmap.c    | 44 ++++++++++++++++++++++++++++++++++--------
>   3 files changed, 86 insertions(+), 12 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 192143b5cdc0..698091c74dbb 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -81,13 +81,17 @@
>    * currently expect (see CONFIG_HAVE_GIGANTIC_FOLIOS): with hugetlb, we expect
>    * no folios larger than 16 GiB on 64bit and 1 GiB on 32bit.
>    */
> -#define MAX_FOLIO_ORDER		get_order(IS_ENABLED(CONFIG_64BIT) ? SZ_16G : SZ_1G)
> +#ifdef CONFIG_64BIT
> +#define MAX_FOLIO_ORDER		(ilog2(SZ_16G) - PAGE_SHIFT)
> +#else
> +#define MAX_FOLIO_ORDER		(ilog2(SZ_1G) - PAGE_SHIFT)
> +#endif
>   #else
>   /*
>    * Without hugetlb, gigantic folios that are bigger than a single PUD are
>    * currently impossible.
>    */
> -#define MAX_FOLIO_ORDER		PUD_ORDER
> +#define MAX_FOLIO_ORDER		(PUD_SHIFT - PAGE_SHIFT)
>   #endif
>   
>   #define MAX_FOLIO_NR_PAGES	(1UL << MAX_FOLIO_ORDER)
> @@ -1402,6 +1406,13 @@ struct memory_failure_stats {
>   };
>   #endif
>   
> +/*
> + * vmemmap optimization (like HVO) is only possible for page orders that fill
> + * two or more pages with struct pages.
> + */
> +#define VMEMMAP_TAIL_MIN_ORDER (ilog2(2 * PAGE_SIZE / sizeof(struct page)))
> +#define NR_VMEMMAP_TAILS (MAX_FOLIO_ORDER - VMEMMAP_TAIL_MIN_ORDER + 1)
> +
>   /*
>    * On NUMA machines, each NUMA node would have a pg_data_t to describe
>    * it's memory layout. On UMA machines there is a single pglist_data which
> @@ -1550,6 +1561,9 @@ typedef struct pglist_data {
>   #ifdef CONFIG_MEMORY_FAILURE
>   	struct memory_failure_stats mf_stats;
>   #endif
> +#ifdef CONFIG_SPARSEMEM_VMEMMAP
> +	unsigned long vmemmap_tails[NR_VMEMMAP_TAILS];

We should record "struct page" instead of pfn, I'll explain below.

> +#endif
>   } pg_data_t;
>   
>   #define node_present_pages(nid)	(NODE_DATA(nid)->node_present_pages)
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index a39a301e08b9..f5f42b92dd7d 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -19,6 +19,7 @@
>   
>   #include <asm/tlbflush.h>
>   #include "hugetlb_vmemmap.h"
> +#include "internal.h"
>   
>   /**
>    * struct vmemmap_remap_walk - walk vmemmap page table
> @@ -505,6 +506,34 @@ static bool vmemmap_should_optimize_folio(const struct hstate *h, struct folio *
>   	return true;
>   }
>   
> +static struct page *vmemmap_get_tail(unsigned int order, int node)
> +{
> +	unsigned long pfn;
> +	unsigned int idx;
> +	struct page *tail, *p;
> +
> +	idx = order - VMEMMAP_TAIL_MIN_ORDER;
> +	pfn = READ_ONCE(NODE_DATA(node)->vmemmap_tails[idx]);
> +	if (pfn)

You’ve assumed that a valid PFN can never be zero, but that
isn’t guaranteed.  If we store the `struct page` pointer
instead, the issue disappears: its virtual address is never
NULL.

Moreover, we only convert back and forth with pfn_to_page()/page_to_pfn();
we never dereference any member of the structure, so we don’t
have to care whether `struct page` has been initialized yet
during early boot (it is safe for us to get page in sparse-vmemmap.c).

> +		return pfn_to_page(pfn);
> +
> +	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++)
> +		prep_compound_tail(p + i, NULL, order);
> +
> +	pfn = PHYS_PFN(virt_to_phys(p));
> +	if (cmpxchg(&NODE_DATA(node)->vmemmap_tails[idx], 0, pfn)) {
> +		__free_page(tail);
> +		pfn = READ_ONCE(NODE_DATA(node)->vmemmap_tails[idx]);
> +	}
> +
> +	return pfn_to_page(pfn);
> +}
> +
>   static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
>   					    struct folio *folio,
>   					    struct list_head *vmemmap_pages,
> @@ -520,6 +549,11 @@ static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
>   	if (!vmemmap_should_optimize_folio(h, folio))
>   		return ret;
>   
> +	nid = folio_nid(folio);
> +	vmemmap_tail = vmemmap_get_tail(h->order, nid);
> +	if (!vmemmap_tail)
> +		return -ENOMEM;
> +
>   	static_branch_inc(&hugetlb_optimize_vmemmap_key);
>   
>   	if (flags & VMEMMAP_SYNCHRONIZE_RCU)
> @@ -537,7 +571,6 @@ static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
>   	 */
>   	folio_set_hugetlb_vmemmap_optimized(folio);
>   
> -	nid = folio_nid(folio);
>   	vmemmap_head = alloc_pages_node(nid, GFP_KERNEL, 0);
>   	if (!vmemmap_head) {
>   		ret = -ENOMEM;
> @@ -548,7 +581,6 @@ static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
>   	list_add(&vmemmap_head->lru, vmemmap_pages);
>   	memmap_pages_add(1);
>   
> -	vmemmap_tail	= vmemmap_head;
>   	vmemmap_start	= (unsigned long)&folio->page;
>   	vmemmap_end	= vmemmap_start + hugetlb_vmemmap_size(h);
>   
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index 37522d6cb398..23abd06f1a4e 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -378,16 +378,45 @@ void vmemmap_wrprotect_hvo(unsigned long addr, unsigned long end,
>   	}
>   }
>   
> -/*
> - * Populate vmemmap pages HVO-style. The first page contains the head
> - * page and needed tail pages, the other ones are mirrors of the first
> - * page.
> - */
> +static __meminit unsigned long vmemmap_get_tail(unsigned int order, int node)
> +{
> +	unsigned long pfn;
> +	unsigned int idx;
> +	struct page *p;
> +
> +	BUG_ON(order < VMEMMAP_TAIL_MIN_ORDER);
> +	BUG_ON(order > MAX_FOLIO_ORDER);
> +
> +	idx = order - VMEMMAP_TAIL_MIN_ORDER;
> +	pfn =  NODE_DATA(node)->vmemmap_tails[idx];
              ^
Why you added a space here?

> +	if (pfn)
> +		return pfn;
> +
> +	p = vmemmap_alloc_block_zero(PAGE_SIZE, node);
> +	if (!p)
> +		return 0;
> +
> +	for (int i = 0; i < PAGE_SIZE / sizeof(struct page); i++)
> +		prep_compound_tail(p + i, NULL, order);
> +
> +	pfn = PHYS_PFN(virt_to_phys(p));
> +	NODE_DATA(node)->vmemmap_tails[idx] = pfn;
> +
> +	return pfn;
> +}
> +
>   int __meminit vmemmap_populate_hvo(unsigned long addr, unsigned long end,
>   				       int node, unsigned long headsize)
>   {
> +	unsigned long maddr, len, tail_pfn;
> +	unsigned int order;
>   	pte_t *pte;
> -	unsigned long maddr;
> +
> +	len = end - addr;
> +	order = ilog2(len * sizeof(struct page) / PAGE_SIZE);
> +	tail_pfn = vmemmap_get_tail(order, node);
> +	if (!tail_pfn)
> +		return -ENOMEM;
>   
>   	for (maddr = addr; maddr < addr + headsize; maddr += PAGE_SIZE) {
>   		pte = vmemmap_populate_address(maddr, node, NULL, -1, 0);
> @@ -398,8 +427,7 @@ int __meminit vmemmap_populate_hvo(unsigned long addr, unsigned long end,
>   	/*
>   	 * Reuse the last page struct page mapped above for the rest.
>   	 */
> -	return vmemmap_populate_range(maddr, end, node, NULL,
> -					pte_pfn(ptep_get(pte)), 0);
> +	return vmemmap_populate_range(maddr, end, node, NULL, tail_pfn, 0);
>   }
>   
>   void __weak __meminit vmemmap_set_pmd(pmd_t *pmd, void *p, int node,

[PATCHv5 12/17] mm: Drop fake head checks

[Kiryl Shutsemau]

With fake head pages eliminated in the previous commit, remove the
supporting infrastructure:

  - page_fixed_fake_head(): no longer needed to detect fake heads;
  - page_is_fake_head(): no longer needed;
  - page_count_writable(): no longer needed for RCU protection;
  - RCU read_lock in page_ref_add_unless(): no longer needed;

This substantially simplifies compound_head() and page_ref_add_unless(),
removing both branches and RCU overhead from these hot paths.

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
---
 include/linux/page-flags.h | 93 ++------------------------------------
 include/linux/page_ref.h   |  8 +---
 2 files changed, 4 insertions(+), 97 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 8f2c7fbc739b..5a8f6fab2255 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -221,102 +221,15 @@ static __always_inline bool compound_info_has_mask(void)
 	return is_power_of_2(sizeof(struct page));
 }
 
-#ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
 DECLARE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key);
 
-/*
- * Return the real head page struct iff the @page is a fake head page, otherwise
- * return the @page itself. See Documentation/mm/vmemmap_dedup.rst.
- */
-static __always_inline const struct page *page_fixed_fake_head(const struct page *page)
-{
-	if (!static_branch_unlikely(&hugetlb_optimize_vmemmap_key))
-		return page;
-
-	/* Fake heads only exists if compound_info_has_mask() is true */
-	if (!compound_info_has_mask())
-		return page;
-
-	/*
-	 * Only addresses aligned with PAGE_SIZE of struct page may be fake head
-	 * struct page. The alignment check aims to avoid access the fields (
-	 * e.g. compound_info) of the @page[1]. It can avoid touch a (possibly)
-	 * cold cacheline in some cases.
-	 */
-	if (IS_ALIGNED((unsigned long)page, PAGE_SIZE) &&
-	    test_bit(PG_head, &page->flags.f)) {
-		/*
-		 * We can safely access the field of the @page[1] with PG_head
-		 * because the @page is a compound page composed with at least
-		 * two contiguous pages.
-		 */
-		unsigned long info = READ_ONCE(page[1].compound_info);
-
-		/* See set_compound_head() */
-		if (likely(info & 1)) {
-			unsigned long p = (unsigned long)page;
-
-			return (const struct page *)(p & info);
-		}
-	}
-	return page;
-}
-
-static __always_inline bool page_count_writable(const struct page *page, int u)
-{
-	if (!static_branch_unlikely(&hugetlb_optimize_vmemmap_key))
-		return true;
-
-	/*
-	 * The refcount check is ordered before the fake-head check to prevent
-	 * the following race:
-	 *   CPU 1 (HVO)                     CPU 2 (speculative PFN walker)
-	 *
-	 *   page_ref_freeze()
-	 *   synchronize_rcu()
-	 *                                   rcu_read_lock()
-	 *                                   page_is_fake_head() is false
-	 *   vmemmap_remap_pte()
-	 *   XXX: struct page[] becomes r/o
-	 *
-	 *   page_ref_unfreeze()
-	 *                                   page_ref_count() is not zero
-	 *
-	 *                                   atomic_add_unless(&page->_refcount)
-	 *                                   XXX: try to modify r/o struct page[]
-	 *
-	 * The refcount check also prevents modification attempts to other (r/o)
-	 * tail pages that are not fake heads.
-	 */
-	if (atomic_read_acquire(&page->_refcount) == u)
-		return false;
-
-	return page_fixed_fake_head(page) == page;
-}
-#else
-static inline const struct page *page_fixed_fake_head(const struct page *page)
-{
-	return page;
-}
-
-static inline bool page_count_writable(const struct page *page, int u)
-{
-	return true;
-}
-#endif
-
-static __always_inline int page_is_fake_head(const struct page *page)
-{
-	return page_fixed_fake_head(page) != page;
-}
-
 static __always_inline unsigned long _compound_head(const struct page *page)
 {
 	unsigned long info = READ_ONCE(page->compound_info);
 
 	/* Bit 0 encodes PageTail() */
 	if (!(info & 1))
-		return (unsigned long)page_fixed_fake_head(page);
+		return (unsigned long)page;
 
 	/*
 	 * If compound_info_has_mask() is false, the rest of compound_info is
@@ -397,7 +310,7 @@ static __always_inline void clear_compound_head(struct page *page)
 
 static __always_inline int PageTail(const struct page *page)
 {
-	return READ_ONCE(page->compound_info) & 1 || page_is_fake_head(page);
+	return READ_ONCE(page->compound_info) & 1;
 }
 
 static __always_inline int PageCompound(const struct page *page)
@@ -924,7 +837,7 @@ static __always_inline bool folio_test_head(const struct folio *folio)
 static __always_inline int PageHead(const struct page *page)
 {
 	PF_POISONED_CHECK(page);
-	return test_bit(PG_head, &page->flags.f) && !page_is_fake_head(page);
+	return test_bit(PG_head, &page->flags.f);
 }
 
 __SETPAGEFLAG(Head, head, PF_ANY)
diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
index 544150d1d5fd..490d0ad6e56d 100644
--- a/include/linux/page_ref.h
+++ b/include/linux/page_ref.h
@@ -230,13 +230,7 @@ static inline int folio_ref_dec_return(struct folio *folio)
 
 static inline bool page_ref_add_unless(struct page *page, int nr, int u)
 {
-	bool ret = false;
-
-	rcu_read_lock();
-	/* avoid writing to the vmemmap area being remapped */
-	if (page_count_writable(page, u))
-		ret = atomic_add_unless(&page->_refcount, nr, u);
-	rcu_read_unlock();
+	bool ret = atomic_add_unless(&page->_refcount, nr, u);
 
 	if (page_ref_tracepoint_active(page_ref_mod_unless))
 		__page_ref_mod_unless(page, nr, ret);
-- 
2.51.2

[PATCHv5 13/17] hugetlb: Remove VMEMMAP_SYNCHRONIZE_RCU

[Kiryl Shutsemau]

The VMEMMAP_SYNCHRONIZE_RCU flag triggered synchronize_rcu() calls to
prevent a race between HVO remapping and page_ref_add_unless(). The
race could occur when a speculative PFN walker tried to modify the
refcount on a struct page that was in the process of being remapped
to a fake head.

With fake heads eliminated, page_ref_add_unless() no longer needs RCU
protection.

Remove the flag and synchronize_rcu() calls.

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
---
 mm/hugetlb_vmemmap.c | 20 ++++----------------
 1 file changed, 4 insertions(+), 16 deletions(-)

diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index f5f42b92dd7d..fd74a164c494 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -47,8 +47,6 @@ struct vmemmap_remap_walk {
 #define VMEMMAP_SPLIT_NO_TLB_FLUSH	BIT(0)
 /* Skip the TLB flush when we remap the PTE */
 #define VMEMMAP_REMAP_NO_TLB_FLUSH	BIT(1)
-/* synchronize_rcu() to avoid writes from page_ref_add_unless() */
-#define VMEMMAP_SYNCHRONIZE_RCU		BIT(2)
 	unsigned long		flags;
 };
 
@@ -409,9 +407,6 @@ static int __hugetlb_vmemmap_restore_folio(const struct hstate *h,
 	if (!folio_test_hugetlb_vmemmap_optimized(folio))
 		return 0;
 
-	if (flags & VMEMMAP_SYNCHRONIZE_RCU)
-		synchronize_rcu();
-
 	vmemmap_start	= (unsigned long)&folio->page;
 	vmemmap_end	= vmemmap_start + hugetlb_vmemmap_size(h);
 
@@ -444,7 +439,7 @@ static int __hugetlb_vmemmap_restore_folio(const struct hstate *h,
  */
 int hugetlb_vmemmap_restore_folio(const struct hstate *h, struct folio *folio)
 {
-	return __hugetlb_vmemmap_restore_folio(h, folio, VMEMMAP_SYNCHRONIZE_RCU);
+	return __hugetlb_vmemmap_restore_folio(h, folio, 0);
 }
 
 /**
@@ -467,14 +462,11 @@ long hugetlb_vmemmap_restore_folios(const struct hstate *h,
 	struct folio *folio, *t_folio;
 	long restored = 0;
 	long ret = 0;
-	unsigned long flags = VMEMMAP_REMAP_NO_TLB_FLUSH | VMEMMAP_SYNCHRONIZE_RCU;
+	unsigned long flags = VMEMMAP_REMAP_NO_TLB_FLUSH;
 
 	list_for_each_entry_safe(folio, t_folio, folio_list, lru) {
 		if (folio_test_hugetlb_vmemmap_optimized(folio)) {
 			ret = __hugetlb_vmemmap_restore_folio(h, folio, flags);
-			/* only need to synchronize_rcu() once for each batch */
-			flags &= ~VMEMMAP_SYNCHRONIZE_RCU;
-
 			if (ret)
 				break;
 			restored++;
@@ -556,8 +548,6 @@ static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
 
 	static_branch_inc(&hugetlb_optimize_vmemmap_key);
 
-	if (flags & VMEMMAP_SYNCHRONIZE_RCU)
-		synchronize_rcu();
 	/*
 	 * Very Subtle
 	 * If VMEMMAP_REMAP_NO_TLB_FLUSH is set, TLB flushing is not performed
@@ -615,7 +605,7 @@ void hugetlb_vmemmap_optimize_folio(const struct hstate *h, struct folio *folio)
 {
 	LIST_HEAD(vmemmap_pages);
 
-	__hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, VMEMMAP_SYNCHRONIZE_RCU);
+	__hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, 0);
 	free_vmemmap_page_list(&vmemmap_pages);
 }
 
@@ -643,7 +633,7 @@ static void __hugetlb_vmemmap_optimize_folios(struct hstate *h,
 	struct folio *folio;
 	int nr_to_optimize;
 	LIST_HEAD(vmemmap_pages);
-	unsigned long flags = VMEMMAP_REMAP_NO_TLB_FLUSH | VMEMMAP_SYNCHRONIZE_RCU;
+	unsigned long flags = VMEMMAP_REMAP_NO_TLB_FLUSH;
 
 	nr_to_optimize = 0;
 	list_for_each_entry(folio, folio_list, lru) {
@@ -696,8 +686,6 @@ static void __hugetlb_vmemmap_optimize_folios(struct hstate *h,
 		int ret;
 
 		ret = __hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, flags);
-		/* only need to synchronize_rcu() once for each batch */
-		flags &= ~VMEMMAP_SYNCHRONIZE_RCU;
 
 		/*
 		 * Pages to be freed may have been accumulated.  If we
-- 
2.51.2

[PATCHv5 14/17] mm/hugetlb: Remove hugetlb_optimize_vmemmap_key static key

[Kiryl Shutsemau]

The hugetlb_optimize_vmemmap_key static key was used to guard fake head
detection in compound_head() and related functions. It allowed skipping
the fake head checks entirely when HVO was not in use.

With fake heads eliminated and the detection code removed, the static
key serves no purpose. Remove its definition and all increment/decrement
calls.

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
---
 include/linux/page-flags.h |  2 --
 mm/hugetlb_vmemmap.c       | 14 ++------------
 2 files changed, 2 insertions(+), 14 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 5a8f6fab2255..1aaa604f4b9b 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -221,8 +221,6 @@ static __always_inline bool compound_info_has_mask(void)
 	return is_power_of_2(sizeof(struct page));
 }
 
-DECLARE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key);
-
 static __always_inline unsigned long _compound_head(const struct page *page)
 {
 	unsigned long info = READ_ONCE(page->compound_info);
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index fd74a164c494..63e7ca85c8c9 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -385,9 +385,6 @@ static int vmemmap_remap_alloc(unsigned long start, unsigned long end,
 	return vmemmap_remap_range(start, end, &walk);
 }
 
-DEFINE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key);
-EXPORT_SYMBOL(hugetlb_optimize_vmemmap_key);
-
 static bool vmemmap_optimize_enabled = IS_ENABLED(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON);
 static int __init hugetlb_vmemmap_optimize_param(char *buf)
 {
@@ -419,10 +416,8 @@ static int __hugetlb_vmemmap_restore_folio(const struct hstate *h,
 	 * discarded vmemmap pages must be allocated and remapping.
 	 */
 	ret = vmemmap_remap_alloc(vmemmap_start, vmemmap_end, flags);
-	if (!ret) {
+	if (!ret)
 		folio_clear_hugetlb_vmemmap_optimized(folio);
-		static_branch_dec(&hugetlb_optimize_vmemmap_key);
-	}
 
 	return ret;
 }
@@ -546,8 +541,6 @@ static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
 	if (!vmemmap_tail)
 		return -ENOMEM;
 
-	static_branch_inc(&hugetlb_optimize_vmemmap_key);
-
 	/*
 	 * Very Subtle
 	 * If VMEMMAP_REMAP_NO_TLB_FLUSH is set, TLB flushing is not performed
@@ -583,10 +576,8 @@ static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
 				 vmemmap_head, vmemmap_tail,
 				 vmemmap_pages, flags);
 out:
-	if (ret) {
-		static_branch_dec(&hugetlb_optimize_vmemmap_key);
+	if (ret)
 		folio_clear_hugetlb_vmemmap_optimized(folio);
-	}
 
 	return ret;
 }
@@ -652,7 +643,6 @@ static void __hugetlb_vmemmap_optimize_folios(struct hstate *h,
 			register_page_bootmem_memmap(pfn_to_section_nr(spfn),
 					&folio->page,
 					HUGETLB_VMEMMAP_RESERVE_SIZE);
-			static_branch_inc(&hugetlb_optimize_vmemmap_key);
 			continue;
 		}
 
-- 
2.51.2

[PATCHv5 15/17] mm: Remove the branch from compound_head()

[Kiryl Shutsemau]

The compound_head() function is a hot path. For example, the zap path
calls it for every leaf page table entry.

Rewrite the helper function in a branchless manner to eliminate the risk
of CPU branch misprediction.

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Zi Yan <ziy@nvidia.com>
---
 include/linux/page-flags.h | 27 +++++++++++++++++----------
 1 file changed, 17 insertions(+), 10 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 1aaa604f4b9b..16384cb6f962 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -224,25 +224,32 @@ static __always_inline bool compound_info_has_mask(void)
 static __always_inline unsigned long _compound_head(const struct page *page)
 {
 	unsigned long info = READ_ONCE(page->compound_info);
+	unsigned long mask;
+
+	if (!compound_info_has_mask()) {
+		/* Bit 0 encodes PageTail() */
+		if (info & 1)
+			return info - 1;
 
-	/* Bit 0 encodes PageTail() */
-	if (!(info & 1))
 		return (unsigned long)page;
-
-	/*
-	 * If compound_info_has_mask() is false, the rest of compound_info is
-	 * the pointer to the head page.
-	 */
-	if (!compound_info_has_mask())
-		return info - 1;
+	}
 
 	/*
 	 * If compoun_info_has_mask() is true the rest of the info encodes
 	 * the mask that converts the address of the tail page to the head page.
 	 *
 	 * No need to clear bit 0 in the mask as 'page' always has it clear.
+	 *
+	 * Let's do it in a branchless manner.
 	 */
-	return (unsigned long)page & info;
+
+	/* Non-tail: -1UL, Tail: 0 */
+	mask = (info & 1) - 1;
+
+	/* Non-tail: -1UL, Tail: info */
+	mask |= info;
+
+	return (unsigned long)page & mask;
 }
 
 #define compound_head(page)	((typeof(page))_compound_head(page))
-- 
2.51.2

[PATCHv5 16/17] hugetlb: Update vmemmap_dedup.rst

[Kiryl Shutsemau]

Update the documentation regarding vmemmap optimization for hugetlb to
reflect the changes in how the kernel maps the tail pages.

Fake heads no longer exist. Remove their description.

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
---
 Documentation/mm/vmemmap_dedup.rst | 60 +++++++++++++-----------------
 1 file changed, 26 insertions(+), 34 deletions(-)

diff --git a/Documentation/mm/vmemmap_dedup.rst b/Documentation/mm/vmemmap_dedup.rst
index 1863d88d2dcb..fca9d0ce282a 100644
--- a/Documentation/mm/vmemmap_dedup.rst
+++ b/Documentation/mm/vmemmap_dedup.rst
@@ -124,33 +124,35 @@ Here is how things look before optimization::
  |           |
  +-----------+
 
-The value of page->compound_info is the same for all tail pages. The first
-page of ``struct page`` (page 0) associated with the HugeTLB page contains the 4
-``struct page`` necessary to describe the HugeTLB. The only use of the remaining
-pages of ``struct page`` (page 1 to page 7) is to point to page->compound_info.
-Therefore, we can remap pages 1 to 7 to page 0. Only 1 page of ``struct page``
-will be used for each HugeTLB page. This will allow us to free the remaining
-7 pages to the buddy allocator.
+The first page of ``struct page`` (page 0) associated with the HugeTLB page
+contains the 4 ``struct page`` necessary to describe the HugeTLB. The remaining
+pages of ``struct page`` (page 1 to page 7) are tail pages.
+
+The optimization is only applied when the size of the struct page is a power-of-2
+In this case, all tail pages of the same order are identical. See
+compound_head(). This allows us to remap the tail pages of the vmemmap to a
+shared, read-only page. The head page is also remapped to a new page. This
+allows the original vmemmap pages to be freed.
 
 Here is how things look after remapping::
 
-    HugeTLB                  struct pages(8 pages)         page frame(8 pages)
- +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+
- |           |                     |     0     | -------------> |     0     |
- |           |                     +-----------+                +-----------+
- |           |                     |     1     | ---------------^ ^ ^ ^ ^ ^ ^
- |           |                     +-----------+                  | | | | | |
- |           |                     |     2     | -----------------+ | | | | |
- |           |                     +-----------+                    | | | | |
- |           |                     |     3     | -------------------+ | | | |
- |           |                     +-----------+                      | | | |
- |           |                     |     4     | ---------------------+ | | |
- |    PMD    |                     +-----------+                        | | |
- |   level   |                     |     5     | -----------------------+ | |
- |  mapping  |                     +-----------+                          | |
- |           |                     |     6     | -------------------------+ |
- |           |                     +-----------+                            |
- |           |                     |     7     | ---------------------------+
+    HugeTLB                  struct pages(8 pages)                 page frame
+ +-----------+ ---virt_to_page---> +-----------+   mapping to   +----------------+
+ |           |                     |     0     | -------------> |       0        |
+ |           |                     +-----------+                +----------------+
+ |           |                     |     1     | ------┐
+ |           |                     +-----------+       |
+ |           |                     |     2     | ------┼        +----------------------------+
+ |           |                     +-----------+       |        | A single, per-node page    |
+ |           |                     |     3     | ------┼------> | frame shared among all     |
+ |           |                     +-----------+       |        | hugepages of the same size |
+ |           |                     |     4     | ------┼        +----------------------------+
+ |           |                     +-----------+       |
+ |           |                     |     5     | ------┼
+ |    PMD    |                     +-----------+       |
+ |   level   |                     |     6     | ------┼
+ |  mapping  |                     +-----------+       |
+ |           |                     |     7     | ------┘
  |           |                     +-----------+
  |           |
  |           |
@@ -172,16 +174,6 @@ The contiguous bit is used to increase the mapping size at the pmd and pte
 (last) level. So this type of HugeTLB page can be optimized only when its
 size of the ``struct page`` structs is greater than **1** page.
 
-Notice: The head vmemmap page is not freed to the buddy allocator and all
-tail vmemmap pages are mapped to the head vmemmap page frame. So we can see
-more than one ``struct page`` struct with ``PG_head`` (e.g. 8 per 2 MB HugeTLB
-page) associated with each HugeTLB page. The ``compound_head()`` can handle
-this correctly. There is only **one** head ``struct page``, the tail
-``struct page`` with ``PG_head`` are fake head ``struct page``.  We need an
-approach to distinguish between those two different types of ``struct page`` so
-that ``compound_head()`` can return the real head ``struct page`` when the
-parameter is the tail ``struct page`` but with ``PG_head``.
-
 Device DAX
 ==========
 
-- 
2.51.2

[PATCHv5 17/17] mm/slab: Use compound_head() in page_slab()

[Kiryl Shutsemau]

page_slab() contained an open-coded implementation of compound_head().

Replace the duplicated code with a direct call to compound_head().

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>

---

I am not sure if this open-coded version is intentional and required for
memdesc transition. Drop the patch if it is.
---
 mm/slab.h | 14 +-------------
 1 file changed, 1 insertion(+), 13 deletions(-)

diff --git a/mm/slab.h b/mm/slab.h
index f68c3ac8126f..970a13ac5b8e 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -137,19 +137,7 @@ static_assert(IS_ALIGNED(offsetof(struct slab, freelist), sizeof(struct freelist
  */
 static inline struct slab *page_slab(const struct page *page)
 {
-	unsigned long info;
-
-	info = READ_ONCE(page->compound_info);
-	if (info & 1) {
-		/* See compound_head() */
-		if (compound_info_has_mask()) {
-			unsigned long p = (unsigned long)page;
-			page = (struct page *)(p & info);
-		} else {
-			page = (struct page *)(info - 1);
-		}
-	}
-
+	page = compound_head(page);
 	if (data_race(page->page_type >> 24) != PGTY_slab)
 		page = NULL;
 
-- 
2.51.2