[PATCH] ublk: remove segment count and size limits

[PATCH] ublk: remove segment count and size limits

[Uday Shankar]

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>
---
 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

Re: [PATCH] ublk: remove segment count and size limits

[Ming Lei]

On Mon, Apr 29, 2024 at 06:53:31PM -0600, Uday Shankar wrote:
> 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>
> ---
>  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;

Reviewed-by: Ming Lei <ming.lei@redhat.com>

Thanks,
Ming

Re: [PATCH] ublk: remove segment count and size limits

[Christoph Hellwig]

On Mon, Apr 29, 2024 at 06:53:31PM -0600, Uday Shankar wrote:
> +	lim.max_segments = USHRT_MAX;
> +	lim.max_segment_size = UINT_MAX;

Please initialize thes at the top of the function as part of the
declaration for lim, like all the other unconditional limits.