[PATCH 7/9] mm: Add deferred_list page flag

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

Stored in the first tail page's flags, this flag replaces the destructor.
That removes the last of the destructors, so remove all references to
folio_dtor and compound_dtor.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 Documentation/admin-guide/kdump/vmcoreinfo.rst |  4 ++--
 include/linux/mm.h                             | 14 --------------
 include/linux/mm_types.h                       |  2 --
 include/linux/page-flags.h                     |  6 ++++++
 kernel/crash_core.c                            |  1 -
 mm/huge_memory.c                               |  4 ++--
 mm/internal.h                                  |  1 -
 mm/page_alloc.c                                |  7 +------
 8 files changed, 11 insertions(+), 28 deletions(-)

diff --git a/Documentation/admin-guide/kdump/vmcoreinfo.rst b/Documentation/admin-guide/kdump/vmcoreinfo.rst
index baa1c355741d..3bd38ac0e7de 100644
--- a/Documentation/admin-guide/kdump/vmcoreinfo.rst
+++ b/Documentation/admin-guide/kdump/vmcoreinfo.rst
@@ -141,8 +141,8 @@ 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_dtor|compound_order|compound_head)
--------------------------------------------------------------------------------------------------
+(page, flags|_refcount|mapping|lru|_mapcount|private|compound_order|compound_head)
+----------------------------------------------------------------------------------
 
 User-space tools compute their values based on the offset of these
 variables. The variables are used when excluding unnecessary pages.
diff --git a/include/linux/mm.h b/include/linux/mm.h
index c8c8b1fd64d3..cf0ae8c51d7f 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1267,20 +1267,6 @@ void folio_copy(struct folio *dst, struct folio *src);
 
 unsigned long nr_free_buffer_pages(void);
 
-/* Compound pages may have a special destructor */
-enum compound_dtor_id {
-	COMPOUND_PAGE_DTOR,
-	TRANSHUGE_PAGE_DTOR,
-	NR_COMPOUND_DTORS
-};
-
-static inline void folio_set_compound_dtor(struct folio *folio,
-		enum compound_dtor_id compound_dtor)
-{
-	VM_BUG_ON_FOLIO(compound_dtor >= NR_COMPOUND_DTORS, folio);
-	folio->_folio_dtor = compound_dtor;
-}
-
 void destroy_large_folio(struct folio *folio);
 
 /* Returns the number of bytes in this potentially compound page. */
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index da538ff68953..d45a2b8041e0 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -282,7 +282,6 @@ static inline struct page *encoded_page_ptr(struct encoded_page *page)
  * @_refcount: Do not access this member directly.  Use folio_ref_count()
  *    to find how many references there are to this folio.
  * @memcg_data: Memory Control Group data.
- * @_folio_dtor: Which destructor to use for this folio.
  * @_folio_order: Do not use directly, call folio_order().
  * @_entire_mapcount: Do not use directly, call folio_entire_mapcount().
  * @_nr_pages_mapped: Do not use directly, call folio_mapcount().
@@ -336,7 +335,6 @@ struct folio {
 			unsigned long _flags_1;
 			unsigned long _head_1;
 	/* public: */
-			unsigned char _folio_dtor;
 			unsigned char _folio_order;
 			atomic_t _entire_mapcount;
 			atomic_t _nr_pages_mapped;
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 01d98695e79f..aabf50dc71a3 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -183,6 +183,9 @@ enum pageflags {
 	/* Is a hugetlb page.  Stored in first tail page. */
 	PG_hugetlb = PG_writeback,
 
+	/* Has a deferred list (may be empty).  First tail page. */
+	PG_deferred_list = PG_reclaim,
+
 	/* non-lru isolated movable page */
 	PG_isolated = PG_reclaim,
 
@@ -809,6 +812,9 @@ static inline void ClearPageCompound(struct page *page)
 	BUG_ON(!PageHead(page));
 	ClearPageHead(page);
 }
+PAGEFLAG(DeferredList, deferred_list, PF_SECOND)
+#else
+TESTPAGEFLAG_FALSE(DeferredList, deferred_list)
 #endif
 
 #define PG_head_mask ((1UL << PG_head))
diff --git a/kernel/crash_core.c b/kernel/crash_core.c
index dd5f87047d06..934dd86e19f5 100644
--- a/kernel/crash_core.c
+++ b/kernel/crash_core.c
@@ -455,7 +455,6 @@ static int __init crash_save_vmcoreinfo_init(void)
 	VMCOREINFO_OFFSET(page, lru);
 	VMCOREINFO_OFFSET(page, _mapcount);
 	VMCOREINFO_OFFSET(page, private);
-	VMCOREINFO_OFFSET(folio, _folio_dtor);
 	VMCOREINFO_OFFSET(folio, _folio_order);
 	VMCOREINFO_OFFSET(page, compound_head);
 	VMCOREINFO_OFFSET(pglist_data, node_zones);
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 99e36ad540c4..3b5db99eb7ac 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -583,7 +583,7 @@ void prep_transhuge_page(struct page *page)
 
 	VM_BUG_ON_FOLIO(folio_order(folio) < 2, folio);
 	INIT_LIST_HEAD(&folio->_deferred_list);
-	folio_set_compound_dtor(folio, TRANSHUGE_PAGE_DTOR);
+	folio_set_deferred_list(folio);
 }
 
 static inline bool is_transparent_hugepage(struct page *page)
@@ -595,7 +595,7 @@ static inline bool is_transparent_hugepage(struct page *page)
 
 	folio = page_folio(page);
 	return is_huge_zero_page(&folio->page) ||
-	       folio->_folio_dtor == TRANSHUGE_PAGE_DTOR;
+		folio_test_deferred_list(folio);
 }
 
 static unsigned long __thp_get_unmapped_area(struct file *filp,
diff --git a/mm/internal.h b/mm/internal.h
index 5a03bc4782a2..e3d11119b04e 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -417,7 +417,6 @@ static inline void prep_compound_head(struct page *page, unsigned int order)
 {
 	struct folio *folio = (struct folio *)page;
 
-	folio_set_compound_dtor(folio, COMPOUND_PAGE_DTOR);
 	folio_set_order(folio, order);
 	atomic_set(&folio->_entire_mapcount, -1);
 	atomic_set(&folio->_nr_pages_mapped, 0);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 21af71aea6eb..9fe9209605a5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -582,9 +582,6 @@ static inline void free_the_page(struct page *page, unsigned int order)
  * 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.
  *
- * The first tail page's ->compound_dtor describes how to destroy the
- * compound page.
- *
  * The first tail page's ->compound_order holds the order of allocation.
  * This usage means that zero-order pages may not be compound.
  */
@@ -603,14 +600,12 @@ void prep_compound_page(struct page *page, unsigned int order)
 
 void destroy_large_folio(struct folio *folio)
 {
-	enum compound_dtor_id dtor = folio->_folio_dtor;
-
 	if (folio_test_hugetlb(folio)) {
 		free_huge_page(folio);
 		return;
 	}
 
-	if (folio_test_transhuge(folio) && dtor == TRANSHUGE_PAGE_DTOR)
+	if (folio_test_deferred_list(folio))
 		free_transhuge_folio(folio);
 	mem_cgroup_uncharge(folio);
 	free_the_page(&folio->page, folio_order(folio));
-- 
2.40.1