mm/page-writeback: drop usage of folio_index

folio_index is only needed for mixed usage of page cache and swap cache.
The remaining three caller in page-writeback are for page cache tag
marking. Swap cache space doesn't use tag (explicitly sets
mapping_set_no_writeback_tags), so use folio->index here directly.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Kairui Song <[e-mail]>
Cc: Matthew Wilcox (Oracle) <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/page-writeback.c

···
 #include <linux/sched/rt.h>
 #include <linux/sched/signal.h>
 #include <linux/mm_inline.h>
+#include <linux/shmem_fs.h>
 #include <trace/events/writeback.h>
 
 #include "internal.h"
-#include "swap.h"
 
 /*
  * Sleep at most 200ms at a time in balance_dirty_pages().
···
 {
 	unsigned long flags;
 
+	/*
+	 * Shmem writeback relies on swap, and swap writeback is LRU based,
+	 * not using the dirty mark.
+	 */
+	VM_WARN_ON_ONCE(folio_test_swapcache(folio) || shmem_mapping(mapping));
+
 	xa_lock_irqsave(&mapping->i_pages, flags);
 	if (folio->mapping) {	/* Race with truncate? */
 		WARN_ON_ONCE(warn && !folio_test_uptodate(folio));
 		folio_account_dirtied(folio, mapping);
-		__xa_set_mark(&mapping->i_pages, folio_index(folio),
-				PAGECACHE_TAG_DIRTY);
+		__xa_set_mark(&mapping->i_pages, folio->index,
+			      PAGECACHE_TAG_DIRTY);
 	}
 	xa_unlock_irqrestore(&mapping->i_pages, flags);
 }
···
 
 		xa_lock_irqsave(&mapping->i_pages, flags);
 		ret = folio_xor_flags_has_waiters(folio, 1 << PG_writeback);
-		__xa_clear_mark(&mapping->i_pages, folio_index(folio),
+		__xa_clear_mark(&mapping->i_pages, folio->index,
 					PAGECACHE_TAG_WRITEBACK);
 		if (bdi->capabilities & BDI_CAP_WRITEBACK_ACCT) {
 			struct bdi_writeback *wb = inode_to_wb(inode);
···
 	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
 
 	if (mapping && mapping_use_writeback_tags(mapping)) {
-		XA_STATE(xas, &mapping->i_pages, folio_index(folio));
+		XA_STATE(xas, &mapping->i_pages, folio->index);
 		struct inode *inode = mapping->host;
 		struct backing_dev_info *bdi = inode_to_bdi(inode);
 		unsigned long flags;