ublk: remove segment count and size limits

ublk_drv currently creates block devices with the default max_segments
and max_segment_size limits of BLK_MAX_SEGMENTS (128) and
BLK_MAX_SEGMENT_SIZE (65536) respectively. These defaults can
artificially constrain the I/O size seen by the ublk server - for
example, suppose that the ublk server has configured itself to accept
I/Os up to 1M and the application is also issuing 1M sized I/Os. If the
I/O buffer used by the application is backed by 4K pages, the buffer
could consist of up to 1M / 4K = 256 physically discontiguous segments
(even if the buffer is virtually contiguous). As such, the I/O could
exceed the default max_segments limit and get split. This can cause
unnecessary performance issues if the ublk server is optimized to handle
1M I/Os. The block layer's segment count/size limits exist to model
hardware constraints which don't exist in ublk_drv's case, so just
remove those limits for the block devices created by ublk_drv.

Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Reviewed-by: Riley Thomasson <riley@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20240430211623.2802036-1-ushankar@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: ps-ushankar

drivers/block/ublk_drv.c

@@ -2177,7 +2177,8 @@ static int ublk_ctrl_start_dev(struct ublk_device *ub, struct io_uring_cmd *cmd)
 		.max_hw_sectors		= p->max_sectors,
 		.chunk_sectors		= p->chunk_sectors,
 		.virt_boundary_mask	= p->virt_boundary_mask,
-
+		.max_segments		= USHRT_MAX,
+		.max_segment_size	= UINT_MAX,
 	};
 	struct gendisk *disk;
 	int ret = -EINVAL;