shmem: convert shmem_replace_page() to use folios throughout

Introduce folio_set_swap_entry() to abstract how both folio->private and
swp_entry_t work.  Use swap_address_space() directly instead of
indirecting through folio_mapping().  Include an assertion that the old
folio is not large as we only allocate a single-page folio to replace it. 
Use folio_put_refs() instead of calling folio_put() twice.

Link: https://lkml.kernel.org/r/20220902194653.1739778-9-willy@infradead.orgSigned-off-by: NMatthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>

include/linux/swap.h

@@ -355,6 +355,11 @@ static inline swp_entry_t folio_swap_entry(struct folio *folio)
 	return entry;
 }
 
+static inline void folio_set_swap_entry(struct folio *folio, swp_entry_t entry)
+{
+	folio->private = (void *)entry.val;
+}
+
 /* linux/mm/workingset.c */
 void workingset_age_nonresident(struct lruvec *lruvec, unsigned long nr_pages);
 void *workingset_eviction(struct folio *folio, struct mem_cgroup *target_memcg);

mm/shmem.c

@@ -1560,12 +1560,6 @@ static struct folio *shmem_alloc_folio(gfp_t gfp,
 	return folio;
 }
 
-static struct page *shmem_alloc_page(gfp_t gfp,
-			struct shmem_inode_info *info, pgoff_t index)
-{
-	return &shmem_alloc_folio(gfp, info, index)->page;
-}
-
 static struct folio *shmem_alloc_and_acct_folio(gfp_t gfp, struct inode *inode,
 		pgoff_t index, bool huge)
 {
@@ -1617,51 +1611,49 @@ static bool shmem_should_replace_folio(struct folio *folio, gfp_t gfp)
 static int shmem_replace_page(struct page **pagep, gfp_t gfp,
 				struct shmem_inode_info *info, pgoff_t index)
 {
-	struct page *oldpage, *newpage;
 	struct folio *old, *new;
 	struct address_space *swap_mapping;
 	swp_entry_t entry;
 	pgoff_t swap_index;
 	int error;
 
-	oldpage = *pagep;
-	entry.val = page_private(oldpage);
+	old = page_folio(*pagep);
+	entry = folio_swap_entry(old);
 	swap_index = swp_offset(entry);
-	swap_mapping = page_mapping(oldpage);
+	swap_mapping = swap_address_space(entry);
 
 	/*
 	 * We have arrived here because our zones are constrained, so don't
 	 * limit chance of success by further cpuset and node constraints.
 	 */
 	gfp &= ~GFP_CONSTRAINT_MASK;
-	newpage = shmem_alloc_page(gfp, info, index);
-	if (!newpage)
+	VM_BUG_ON_FOLIO(folio_test_large(old), old);
+	new = shmem_alloc_folio(gfp, info, index);
+	if (!new)
 		return -ENOMEM;
 
-	get_page(newpage);
-	copy_highpage(newpage, oldpage);
-	flush_dcache_page(newpage);
+	folio_get(new);
+	folio_copy(new, old);
+	flush_dcache_folio(new);
 
-	__SetPageLocked(newpage);
-	__SetPageSwapBacked(newpage);
-	SetPageUptodate(newpage);
-	set_page_private(newpage, entry.val);
-	SetPageSwapCache(newpage);
+	__folio_set_locked(new);
+	__folio_set_swapbacked(new);
+	folio_mark_uptodate(new);
+	folio_set_swap_entry(new, entry);
+	folio_set_swapcache(new);
 
 	/*
 	 * Our caller will very soon move newpage out of swapcache, but it's
 	 * a nice clean interface for us to replace oldpage by newpage there.
 	 */
 	xa_lock_irq(&swap_mapping->i_pages);
-	error = shmem_replace_entry(swap_mapping, swap_index, oldpage, newpage);
+	error = shmem_replace_entry(swap_mapping, swap_index, old, new);
 	if (!error) {
-		old = page_folio(oldpage);
-		new = page_folio(newpage);
 		mem_cgroup_migrate(old, new);
-		__inc_lruvec_page_state(newpage, NR_FILE_PAGES);
-		__inc_lruvec_page_state(newpage, NR_SHMEM);
-		__dec_lruvec_page_state(oldpage, NR_FILE_PAGES);
-		__dec_lruvec_page_state(oldpage, NR_SHMEM);
+		__lruvec_stat_mod_folio(new, NR_FILE_PAGES, 1);
+		__lruvec_stat_mod_folio(new, NR_SHMEM, 1);
+		__lruvec_stat_mod_folio(old, NR_FILE_PAGES, -1);
+		__lruvec_stat_mod_folio(old, NR_SHMEM, -1);
 	}
 	xa_unlock_irq(&swap_mapping->i_pages);
 
@@ -1671,18 +1663,17 @@ static int shmem_replace_page(struct page **pagep, gfp_t gfp,
 		 * both PageSwapCache and page_private after getting page lock;
 		 * but be defensive.  Reverse old to newpage for clear and free.
 		 */
-		oldpage = newpage;
+		old = new;
 	} else {
-		lru_cache_add(newpage);
-		*pagep = newpage;
+		folio_add_lru(new);
+		*pagep = &new->page;
 	}
 
-	ClearPageSwapCache(oldpage);
-	set_page_private(oldpage, 0);
+	folio_clear_swapcache(old);
+	old->private = NULL;
 
-	unlock_page(oldpage);
-	put_page(oldpage);
-	put_page(oldpage);
+	folio_unlock(old);
+	folio_put_refs(old, 2);
 	return error;
 }
 
@@ -2383,6 +2374,12 @@ static struct inode *shmem_get_inode(struct super_block *sb, struct inode *dir,
 }
 
 #ifdef CONFIG_USERFAULTFD
+static struct page *shmem_alloc_page(gfp_t gfp,
+			struct shmem_inode_info *info, pgoff_t index)
+{
+	return &shmem_alloc_folio(gfp, info, index)->page;
+}
+
 int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
 			   pmd_t *dst_pmd,
 			   struct vm_area_struct *dst_vma,