mm/swap: use dedicated entry for swap in folio

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

Link: https://lkml.kernel.org/r/20230821160849.531668-3-david@redhat.com
Signed-off-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Chris Li <chrisl@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Matthew Wilcox

include/linux/mm_types.h

@@ -248,6 +248,14 @@ static inline struct page *encoded_page_ptr(struct encoded_page *page)
 	return (struct page *)(~ENCODE_PAGE_BITS & (unsigned long)page);
 }
 
+/*
+ * 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.
@@ -258,7 +266,7 @@ 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()
@@ -301,7 +309,10 @@ 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
@@ -1209,14 +1220,6 @@ 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.

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 */