Re: [PATCH v7 4/6] block: loop: prepare for supporing direct IO

+	/*
+	 * loop block's logical block size is 512, now
+	 * we support direct I/O only if the backing
+	 * block devices' minimize I/O size is 512 and
+	 * the offset is aligned with 512.
+	 */
+	if (dio) {
+		if (inode->i_sb->s_bdev &&
+			bdev_io_min(inode->i_sb->s_bdev) == 512 &&
+			!(lo->lo_offset & 511))

Why the hardcoded value?  I suspect this should be more like:

	if (dio && inode->i_sb->s_bdev &&
	    (lo->lo_offset & (bdev_io_min(inode->i_sb->s_bdev) - 1)) != 0)
		dio = false;

+	blk_mq_freeze_queue(lo->lo_queue);
+	lo->use_dio = use_dio;
+	if (use_dio)
+		lo->lo_flags |= LO_FLAGS_DIRECT_IO;
+	else
+		lo->lo_flags &= ~LO_FLAGS_DIRECT_IO;
+	blk_mq_unfreeze_queue(lo->lo_queue);

Locking?