iomap: use filemap_range_needs_writeback() for O_DIRECT reads

For reads, use the better variant of checking for the need to call
filemap_write_and_wait_range() when doing O_DIRECT.  This avoids falling
back to the slow path for IOCB_NOWAIT, if there are no pages to wait for
(or write out).

Link: https://lkml.kernel.org/r/20210224164455.1096727-4-axboe@kernel.dkSigned-off-by: NJens Axboe <axboe@kernel.dk>
Reviewed-by: NJan Kara <jack@suse.cz>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/iomap/direct-io.c

@@ -487,12 +487,28 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
 		if (pos >= dio->i_size)
 			goto out_free_dio;
 
+		if (iocb->ki_flags & IOCB_NOWAIT) {
+			if (filemap_range_needs_writeback(mapping, pos, end)) {
+				ret = -EAGAIN;
+				goto out_free_dio;
+			}
+			iomap_flags |= IOMAP_NOWAIT;
+		}
+
 		if (iter_is_iovec(iter))
 			dio->flags |= IOMAP_DIO_DIRTY;
 	} else {
 		iomap_flags |= IOMAP_WRITE;
 		dio->flags |= IOMAP_DIO_WRITE;
 
+		if (iocb->ki_flags & IOCB_NOWAIT) {
+			if (filemap_range_has_page(mapping, pos, end)) {
+				ret = -EAGAIN;
+				goto out_free_dio;
+			}
+			iomap_flags |= IOMAP_NOWAIT;
+		}
+
 		/* for data sync or sync, we need sync completion processing */
 		if (iocb->ki_flags & IOCB_DSYNC)
 			dio->flags |= IOMAP_DIO_NEED_SYNC;
@@ -507,14 +523,6 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
 			dio->flags |= IOMAP_DIO_WRITE_FUA;
 	}
 
-	if (iocb->ki_flags & IOCB_NOWAIT) {
-		if (filemap_range_has_page(mapping, pos, end)) {
-			ret = -EAGAIN;
-			goto out_free_dio;
-		}
-		iomap_flags |= IOMAP_NOWAIT;
-	}
-
 	if (dio_flags & IOMAP_DIO_OVERWRITE_ONLY) {
 		ret = -EAGAIN;
 		if (pos >= dio->i_size || pos + count > dio->i_size)