fs/writeback: Convert inode_switch_wbs_work_fn to folios

This gets the statistics correct for large folios by modifying the
counters by the number of pages in the folio instead of by 1.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>

fs/fs-writeback.c

@@ -372,7 +372,7 @@ static bool inode_do_switch_wbs(struct inode *inode,
 {
 	struct address_space *mapping = inode->i_mapping;
 	XA_STATE(xas, &mapping->i_pages, 0);
-	struct page *page;
+	struct folio *folio;
 	bool switched = false;
 
 	spin_lock(&inode->i_lock);
@@ -389,21 +389,23 @@ static bool inode_do_switch_wbs(struct inode *inode,
 
 	/*
 	 * Count and transfer stats.  Note that PAGECACHE_TAG_DIRTY points
-	 * to possibly dirty pages while PAGECACHE_TAG_WRITEBACK points to
-	 * pages actually under writeback.
+	 * to possibly dirty folios while PAGECACHE_TAG_WRITEBACK points to
+	 * folios actually under writeback.
 	 */
-	xas_for_each_marked(&xas, page, ULONG_MAX, PAGECACHE_TAG_DIRTY) {
-		if (PageDirty(page)) {
-			dec_wb_stat(old_wb, WB_RECLAIMABLE);
-			inc_wb_stat(new_wb, WB_RECLAIMABLE);
+	xas_for_each_marked(&xas, folio, ULONG_MAX, PAGECACHE_TAG_DIRTY) {
+		if (folio_test_dirty(folio)) {
+			long nr = folio_nr_pages(folio);
+			wb_stat_mod(old_wb, WB_RECLAIMABLE, -nr);
+			wb_stat_mod(new_wb, WB_RECLAIMABLE, nr);
 		}
 	}
 
 	xas_set(&xas, 0);
-	xas_for_each_marked(&xas, page, ULONG_MAX, PAGECACHE_TAG_WRITEBACK) {
-		WARN_ON_ONCE(!PageWriteback(page));
-		dec_wb_stat(old_wb, WB_WRITEBACK);
-		inc_wb_stat(new_wb, WB_WRITEBACK);
+	xas_for_each_marked(&xas, folio, ULONG_MAX, PAGECACHE_TAG_WRITEBACK) {
+		long nr = folio_nr_pages(folio);
+		WARN_ON_ONCE(!folio_test_writeback(folio));
+		wb_stat_mod(old_wb, WB_WRITEBACK, -nr);
+		wb_stat_mod(new_wb, WB_WRITEBACK, nr);
 	}
 
 	if (mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK)) {