[PATCH v2 10/13] mm: Free up a word in the first tail page

["Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>]

Store the folio order in the low byte of the flags word in the first
tail page.  This frees up the word that was being used to store the
order and dtor bytes previously.

Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
 include/linux/mm.h         | 10 +++++-----
 include/linux/mm_types.h   |  3 +--
 include/linux/page-flags.h |  7 ++++---
 kernel/crash_core.c        |  1 -
 mm/internal.h              |  2 +-
 5 files changed, 11 insertions(+), 12 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index cf0ae8c51d7f..85568e2b2556 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1028,7 +1028,7 @@ struct inode;
  * compound_order() can be called without holding a reference, which means
  * that niceties like page_folio() don't work.  These callers should be
  * prepared to handle wild return values.  For example, PG_head may be
- * set before _folio_order is initialised, or this may be a tail page.
+ * set before the order is initialised, or this may be a tail page.
  * See compaction.c for some good examples.
  */
 static inline unsigned int compound_order(struct page *page)
@@ -1037,7 +1037,7 @@ static inline unsigned int compound_order(struct page *page)
 
 	if (!test_bit(PG_head, &folio->flags))
 		return 0;
-	return folio->_folio_order;
+	return folio->_flags_1 & 0xff;
 }
 
 /**
@@ -1053,7 +1053,7 @@ static inline unsigned int folio_order(struct folio *folio)
 {
 	if (!folio_test_large(folio))
 		return 0;
-	return folio->_folio_order;
+	return folio->_flags_1 & 0xff;
 }
 
 #include <linux/huge_mm.h>
@@ -2025,7 +2025,7 @@ static inline long folio_nr_pages(struct folio *folio)
 #ifdef CONFIG_64BIT
 	return folio->_folio_nr_pages;
 #else
-	return 1L << folio->_folio_order;
+	return 1L << (folio->_flags_1 & 0xff);
 #endif
 }
 
@@ -2043,7 +2043,7 @@ static inline unsigned long compound_nr(struct page *page)
 #ifdef CONFIG_64BIT
 	return folio->_folio_nr_pages;
 #else
-	return 1L << folio->_folio_order;
+	return 1L << (folio->_flags_1 & 0xff);
 #endif
 }
 
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index d45a2b8041e0..659c7b84726c 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_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().
  * @_pincount: Do not use directly, call folio_maybe_dma_pinned().
@@ -334,8 +333,8 @@ struct folio {
 		struct {
 			unsigned long _flags_1;
 			unsigned long _head_1;
+			unsigned long _folio_avail;
 	/* public: */
-			unsigned char _folio_order;
 			atomic_t _entire_mapcount;
 			atomic_t _nr_pages_mapped;
 			atomic_t _pincount;
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index b452fba9bc71..5b466e619f71 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -184,7 +184,8 @@ enum pageflags {
 
 	/*
 	 * Flags only valid for compound pages.  Stored in first tail page's
-	 * flags word.
+	 * flags word.  Cannot use the first 8 flags or any flag marked as
+	 * PF_ANY.
 	 */
 
 	/* At least one page in this folio has the hwpoison flag set */
@@ -1065,8 +1066,8 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page)
  * the CHECK_AT_FREE flags above, so need to be cleared.
  */
 #define PAGE_FLAGS_SECOND						\
-	(1UL << PG_has_hwpoisoned	| 1UL << PG_hugetlb |		\
-	 1UL << PG_large_rmappable)
+	(0xffUL /* order */		| 1UL << PG_has_hwpoisoned |	\
+	 1UL << PG_hugetlb		| 1UL << PG_large_rmappable)
 
 #define PAGE_FLAGS_PRIVATE				\
 	(1UL << PG_private | 1UL << PG_private_2)
diff --git a/kernel/crash_core.c b/kernel/crash_core.c
index 934dd86e19f5..693445e1f7f6 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_order);
 	VMCOREINFO_OFFSET(page, compound_head);
 	VMCOREINFO_OFFSET(pglist_data, node_zones);
 	VMCOREINFO_OFFSET(pglist_data, nr_zones);
diff --git a/mm/internal.h b/mm/internal.h
index 9dc7629ffbc9..5c777b6779fa 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -407,7 +407,7 @@ static inline void folio_set_order(struct folio *folio, unsigned int order)
 	if (WARN_ON_ONCE(!order || !folio_test_large(folio)))
 		return;
 
-	folio->_folio_order = order;
+	folio->_flags_1 = (folio->_flags_1 & ~0xffUL) | order;
 #ifdef CONFIG_64BIT
 	folio->_folio_nr_pages = 1U << order;
 #endif
-- 
2.40.1