mm: remove PageSwapCache

This flag is now only used on folios, so we can remove all the page
accessors and reword the comments that refer to them.

Link: https://lkml.kernel.org/r/20240821193445.2294269-5-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/mm_types.h

@@ -109,7 +109,7 @@ struct page {
 			/**
 			 * @private: Mapping-private opaque data.
 			 * Usually used for buffer_heads if PagePrivate.
-			 * Used for swp_entry_t if PageSwapCache.
+			 * Used for swp_entry_t if swapcache flag set.
 			 * Indicates order in the buddy system if PageBuddy.
 			 */
 			unsigned long private;

include/linux/page-flags.h

@@ -574,15 +574,10 @@ static __always_inline bool folio_test_swapcache(const struct folio *folio)
 			test_bit(PG_swapcache, const_folio_flags(folio, 0));
 }
 
-static __always_inline bool PageSwapCache(const struct page *page)
-{
-	return folio_test_swapcache(page_folio(page));
-}
-
-SETPAGEFLAG(SwapCache, swapcache, PF_NO_TAIL)
-CLEARPAGEFLAG(SwapCache, swapcache, PF_NO_TAIL)
+FOLIO_SET_FLAG(swapcache, FOLIO_HEAD_PAGE)
+FOLIO_CLEAR_FLAG(swapcache, FOLIO_HEAD_PAGE)
 #else
-PAGEFLAG_FALSE(SwapCache, swapcache)
+FOLIO_FLAG_FALSE(swapcache)
 #endif
 
 PAGEFLAG(Unevictable, unevictable, PF_HEAD)

mm/ksm.c

@@ -909,12 +909,13 @@ static struct folio *ksm_get_folio(struct ksm_stable_node *stable_node,
 	 */
 	while (!folio_try_get(folio)) {
 		/*
-		 * Another check for page->mapping != expected_mapping would
-		 * work here too.  We have chosen the !PageSwapCache test to
-		 * optimize the common case, when the page is or is about to
-		 * be freed: PageSwapCache is cleared (under spin_lock_irq)
-		 * in the ref_freeze section of __remove_mapping(); but Anon
-		 * folio->mapping reset to NULL later, in free_pages_prepare().
+		 * Another check for folio->mapping != expected_mapping
+		 * would work here too.  We have chosen to test the
+		 * swapcache flag to optimize the common case, when the
+		 * folio is or is about to be freed: the swapcache flag
+		 * is cleared (under spin_lock_irq) in the ref_freeze
+		 * section of __remove_mapping(); but anon folio->mapping
+		 * is reset to NULL later, in free_pages_prepare().
 		 */
 		if (!folio_test_swapcache(folio))
 			goto stale;
@@ -945,7 +946,7 @@ static struct folio *ksm_get_folio(struct ksm_stable_node *stable_node,
 
 stale:
 	/*
-	 * We come here from above when page->mapping or !PageSwapCache
+	 * We come here from above when folio->mapping or the swapcache flag
 	 * suggests that the node is stale; but it might be under migration.
 	 * We need smp_rmb(), matching the smp_wmb() in folio_migrate_ksm(),
 	 * before checking whether node->kpfn has been changed.
@@ -1452,7 +1453,7 @@ static int try_to_merge_one_page(struct vm_area_struct *vma,
 		goto out;
 
 	/*
-	 * We need the page lock to read a stable PageSwapCache in
+	 * We need the folio lock to read a stable swapcache flag in
 	 * write_protect_page().  We use trylock_page() instead of
 	 * lock_page() because we don't want to wait here - we
 	 * prefer to continue scanning and merging different pages,
@@ -3123,7 +3124,7 @@ void folio_migrate_ksm(struct folio *newfolio, struct folio *folio)
 		 * newfolio->mapping was set in advance; now we need smp_wmb()
 		 * to make sure that the new stable_node->kpfn is visible
 		 * to ksm_get_folio() before it can see that folio->mapping
-		 * has gone stale (or that folio_test_swapcache has been cleared).
+		 * has gone stale (or that the swapcache flag has been cleared).
 		 */
 		smp_wmb();
 		folio_set_stable_node(folio, NULL);

mm/migrate.c

@@ -639,7 +639,8 @@ void folio_migrate_flags(struct folio *newfolio, struct folio *folio)
 	folio_migrate_ksm(newfolio, folio);
 	/*
 	 * Please do not reorder this without considering how mm/ksm.c's
-	 * ksm_get_folio() depends upon ksm_migrate_page() and PageSwapCache().
+	 * ksm_get_folio() depends upon ksm_migrate_page() and the
+	 * swapcache flag.
 	 */
 	if (folio_test_swapcache(folio))
 		folio_clear_swapcache(folio);

mm/shmem.c

@@ -502,8 +502,8 @@ static int shmem_replace_entry(struct address_space *mapping,
  * Sometimes, before we decide whether to proceed or to fail, we must check
  * that an entry was not already brought back from swap by a racing thread.
  *
- * Checking page is not enough: by the time a SwapCache page is locked, it
- * might be reused, and again be SwapCache, using the same swap as before.
+ * Checking folio is not enough: by the time a swapcache folio is locked, it
+ * might be reused, and again be swapcache, using the same swap as before.
  */
 static bool shmem_confirm_swap(struct address_space *mapping,
 			       pgoff_t index, swp_entry_t swap)
@@ -1965,9 +1965,10 @@ static int shmem_replace_folio(struct folio **foliop, gfp_t gfp,
 
 	if (unlikely(error)) {
 		/*
-		 * Is this possible?  I think not, now that our callers check
-		 * both PageSwapCache and page_private after getting page lock;
-		 * but be defensive.  Reverse old to newpage for clear and free.
+		 * Is this possible?  I think not, now that our callers
+		 * check both the swapcache flag and folio->private
+		 * after getting the folio lock; but be defensive.
+		 * Reverse old to newpage for clear and free.
 		 */
 		old = new;
 	} else {