mm: use SWP_SYNCHRONOUS_IO more intelligently

There is no point in trying to call bdev_read_page if SWP_SYNCHRONOUS_IO
is not set, as the device won't support it.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

mm/page_io.c

@@ -403,6 +403,7 @@ int swap_readpage(struct page *page, bool synchronous)
 		goto out;
 	}
 
+	if (sis->flags & SWP_SYNCHRONOUS_IO) {
 		ret = bdev_read_page(sis->bdev, swap_page_sector(page), page);
 		if (!ret) {
 			if (trylock_page(page)) {
@@ -413,6 +414,7 @@ int swap_readpage(struct page *page, bool synchronous)
 			count_vm_event(PSWPIN);
 			goto out;
 		}
+	}
 
 	ret = 0;
 	bio = get_swap_bio(GFP_KERNEL, page, end_swap_bio_read);