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@xxxxxxxxxxxxxxx> Reviewed-by: Riley Thomasson <riley@xxxxxxxxxxxxxxx> --- drivers/block/ublk_drv.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c index bea3d5cf8a83..835b0cc7c032 100644 --- a/drivers/block/ublk_drv.c +++ b/drivers/block/ublk_drv.c @@ -2209,6 +2209,9 @@ static int ublk_ctrl_start_dev(struct ublk_device *ub, struct io_uring_cmd *cmd) lim.max_zone_append_sectors = p->max_zone_append_sectors; } + lim.max_segments = USHRT_MAX; + lim.max_segment_size = UINT_MAX; + if (wait_for_completion_interruptible(&ub->completion) != 0) return -EINTR; -- 2.34.1
