[PATCH mm-unstable v1 2/4] mm/swap: use dedicated entry for swap in folio

[David Hildenbrand <david@xxxxxxxxxx>]

From: Matthew Wilcox <willy@xxxxxxxxxxxxx>

Let's stop working on the private field and use an explicit swap field.
We have to move the swp_entry_t typedef.

Signed-off-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 include/linux/mm_types.h | 23 +++++++++++++----------
 include/linux/swap.h     |  5 ++---
 2 files changed, 15 insertions(+), 13 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 61361f1750c3..438a07854f8c 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -259,6 +259,14 @@ static inline struct page *encoded_page_ptr(struct encoded_page *page)
  */
 #define  TAIL_MAPPING_REUSED_MAX  (2)
 
+/*
+ * A swap entry has to fit into a "unsigned long", as the entry is hidden
+ * in the "index" field of the swapper address space.
+ */
+typedef struct {
+	unsigned long val;
+} swp_entry_t;
+
 /**
  * struct folio - Represents a contiguous set of bytes.
  * @flags: Identical to the page flags.
@@ -269,7 +277,7 @@ static inline struct page *encoded_page_ptr(struct encoded_page *page)
  * @index: Offset within the file, in units of pages.  For anonymous memory,
  *    this is the index from the beginning of the mmap.
  * @private: Filesystem per-folio data (see folio_attach_private()).
- *    Used for swp_entry_t if folio_test_swapcache().
+ * @swap: Used for swp_entry_t if folio_test_swapcache().
  * @_mapcount: Do not access this member directly.  Use folio_mapcount() to
  *    find out how many times this folio is mapped by userspace.
  * @_refcount: Do not access this member directly.  Use folio_ref_count()
@@ -312,7 +320,10 @@ struct folio {
 			};
 			struct address_space *mapping;
 			pgoff_t index;
-			void *private;
+			union {
+				void *private;
+				swp_entry_t swap;
+			};
 			atomic_t _mapcount;
 			atomic_t _refcount;
 #ifdef CONFIG_MEMCG
@@ -1220,14 +1231,6 @@ enum tlb_flush_reason {
 	NR_TLB_FLUSH_REASONS,
 };
 
- /*
-  * A swap entry has to fit into a "unsigned long", as the entry is hidden
-  * in the "index" field of the swapper address space.
-  */
-typedef struct {
-	unsigned long val;
-} swp_entry_t;
-
 /**
  * enum fault_flag - Fault flag definitions.
  * @FAULT_FLAG_WRITE: Fault was a write fault.
diff --git a/include/linux/swap.h b/include/linux/swap.h
index 84fe0e94f5cd..82859a1944f5 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -335,8 +335,7 @@ struct swap_info_struct {
 
 static inline swp_entry_t folio_swap_entry(struct folio *folio)
 {
-	swp_entry_t entry = { .val = page_private(&folio->page) };
-	return entry;
+	return folio->swap;
 }
 
 static inline swp_entry_t page_swap_entry(struct page *page)
@@ -350,7 +349,7 @@ static inline swp_entry_t page_swap_entry(struct page *page)
 
 static inline void folio_set_swap_entry(struct folio *folio, swp_entry_t entry)
 {
-	folio->private = (void *)entry.val;
+	folio->swap = entry;
 }
 
 /* linux/mm/workingset.c */
-- 
2.41.0