btrfs: update some folio related comments

Remove references to the page lock and page->mapping.  Also btrfs folios
can never be swizzled into swap (mentioned in extent_write_cache_pages()).

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -2192,10 +2192,8 @@ static int extent_write_cache_pages(struct address_space *mapping,
 			done_index = folio_next_index(folio);
 			/*
 			 * At this point we hold neither the i_pages lock nor
-			 * the page lock: the page may be truncated or
-			 * invalidated (changing page->mapping to NULL),
-			 * or even swizzled back from swapper_space to
-			 * tmpfs file mapping
+			 * the folio lock: the folio may be truncated or
+			 * invalidated (changing folio->mapping to NULL).
 			 */
 			if (!folio_trylock(folio)) {
 				submit_write_bio(bio_ctrl, 0);
@@ -3233,7 +3231,7 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
 	}
 	/*
 	 * Now all pages of that extent buffer is unmapped, set UNMAPPED flag,
-	 * so it can be cleaned up without utilizing page->mapping.
+	 * so it can be cleaned up without utilizing folio->mapping.
 	 */
 	set_bit(EXTENT_BUFFER_UNMAPPED, &eb->bflags);
 

fs/btrfs/inode.c

@@ -2896,7 +2896,7 @@ int btrfs_writepage_cow_fixup(struct folio *folio)
 	 * We are already holding a reference to this inode from
 	 * write_cache_pages.  We need to hold it because the space reservation
 	 * takes place outside of the folio lock, and we can't trust
-	 * page->mapping outside of the folio lock.
+	 * folio->mapping outside of the folio lock.
 	 */
 	ihold(inode);
 	btrfs_folio_set_checked(fs_info, folio, folio_pos(folio), folio_size(folio));

fs/btrfs/subpage.c

@@ -72,7 +72,7 @@ bool btrfs_is_subpage(const struct btrfs_fs_info *fs_info, struct address_space
 
 	/*
 	 * Only data pages (either through DIO or compression) can have no
-	 * mapping. And if page->mapping->host is data inode, it's subpage.
+	 * mapping. And if mapping->host is data inode, it's subpage.
 	 * As we have ruled our sectorsize >= PAGE_SIZE case already.
 	 */
 	if (!mapping || !mapping->host || is_data_inode(BTRFS_I(mapping->host)))