mm: consolidate dax / non-dax checks for writeback

We have this complex conditional copied to several places. Turn it into
a helper function.
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jeff Layton <jlayton@redhat.com>

mm/filemap.c

@@ -522,12 +522,17 @@ int filemap_fdatawait(struct address_space *mapping)
 }
 EXPORT_SYMBOL(filemap_fdatawait);
 
+static bool mapping_needs_writeback(struct address_space *mapping)
+{
+	return (!dax_mapping(mapping) && mapping->nrpages) ||
+	    (dax_mapping(mapping) && mapping->nrexceptional);
+}
+
 int filemap_write_and_wait(struct address_space *mapping)
 {
 	int err = 0;
 
-	if ((!dax_mapping(mapping) && mapping->nrpages) ||
-	    (dax_mapping(mapping) && mapping->nrexceptional)) {
+	if (mapping_needs_writeback(mapping)) {
 		err = filemap_fdatawrite(mapping);
 		/*
 		 * Even if the above returned error, the pages may be
@@ -566,8 +571,7 @@ int filemap_write_and_wait_range(struct address_space *mapping,
 {
 	int err = 0;
 
-	if ((!dax_mapping(mapping) && mapping->nrpages) ||
-	    (dax_mapping(mapping) && mapping->nrexceptional)) {
+	if (mapping_needs_writeback(mapping)) {
 		err = __filemap_fdatawrite_range(mapping, lstart, lend,
 						 WB_SYNC_ALL);
 		/* See comment of filemap_write_and_wait() */
@@ -656,8 +660,7 @@ int file_write_and_wait_range(struct file *file, loff_t lstart, loff_t lend)
 	int err = 0, err2;
 	struct address_space *mapping = file->f_mapping;
 
-	if ((!dax_mapping(mapping) && mapping->nrpages) ||
-	    (dax_mapping(mapping) && mapping->nrexceptional)) {
+	if (mapping_needs_writeback(mapping)) {
 		err = __filemap_fdatawrite_range(mapping, lstart, lend,
 						 WB_SYNC_ALL);
 		/* See comment of filemap_write_and_wait() */