mm: free up a word in the first tail page

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.

Link: https://lkml.kernel.org/r/20230816151201.3655946-11-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Yanteng Si <siyanteng@loongson.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/mm.h

@@ -1000,7 +1000,7 @@ 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)
@@ -1009,7 +1009,7 @@ 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;
 }
 
 /**
@@ -1025,7 +1025,7 @@ 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>
@@ -1996,7 +1996,7 @@ 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
 }
 
@@ -2014,7 +2014,7 @@ 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
 }
 

include/linux/mm_types.h

@@ -264,7 +264,6 @@ 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().
@@ -316,8 +315,8 @@ 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;

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 */
@@ -1081,8 +1082,8 @@ 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)

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);

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