mm: rename the 'compound_head' field in the 'struct page' to 'compound_info'

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'.

Link: https://lkml.kernel.org/r/20260227194302.274384-4-kas@kernel.org
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand (arm) <david@kernel.org>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: Baoquan He <bhe@redhat.com>
Cc: Christoph Lameter <cl@gentwo.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Frank van der Linden <fvdl@google.com>
Cc: Harry Yoo <harry.yoo@oracle.com>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Usama Arif <usamaarif642@gmail.com>
Cc: WANG Xuerui <kernel@xen0n.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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

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.

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);

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;
 }
@@ -873,12 +873,12 @@ static inline bool folio_test_large(const struct folio *folio)
 static __always_inline void set_compound_head(struct page *tail,
 		const struct page *head, unsigned int order)
 {
-	WRITE_ONCE(tail->compound_head, (unsigned long)head + 1);
+	WRITE_ONCE(tail->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

include/linux/types.h

@@ -239,7 +239,7 @@ 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().
  */

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

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.

mm/slab.h

@@ -94,7 +94,7 @@ 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);

mm/util.c

@@ -1276,7 +1276,7 @@ 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;