Re: [PATCH v2] scsi_debug: Support configuring the maximum segment size

[John Garry <john.g.garry@oracle.com>]

On 25/03/2026 21:27, Bart Van Assche wrote:
> Add a kernel module parameter for configuring the maximum segment size.
> Note: blk_validate_limits() considers zero as the default and changes it
> into a valid value. Values between 1 and BLK_MIN_SEGMENT_SIZE are rejected
> by blk_validate_limits().

I thought that scsi_host_alloc() will change 0 -> BLK_MAX_SEGMENT_SIZE, 
right?

And I would suggest to reject non-zero max_segment_size modparam < 
BLK_MIN_SEGMENT_SIZE

> 
> This patch enables testing SCSI support for segments smaller than the
> page size.
> 
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> ---
> Cc: John Garry <john.g.garry@oracle.com>
> Cc: Doug Gilbert <dgilbert@interlog.com>
> 
> Changes compared to v1:
>   - Removed sdebug_driver_template.max_segment_size.
>   - Changed the default max_segment_size value from BLK_MAX_SEGMENT_SIZE into
>     UINT_MAX (this is the same as the current limit, -1U).
>   - Explained in the patch description that blk_validate_limits() rejects
>     values between 1 and BLK_MIN_SEGMENT_SIZE.
> 
>   drivers/scsi/scsi_debug.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/scsi/scsi_debug.c b/drivers/scsi/scsi_debug.c
> index 1515495fd9ea..44309a16eb68 100644
> --- a/drivers/scsi/scsi_debug.c
> +++ b/drivers/scsi/scsi_debug.c
> @@ -915,6 +915,7 @@ static int sdebug_host_max_queue;	/* per host */
>   static int sdebug_lowest_aligned = DEF_LOWEST_ALIGNED;
>   static int sdebug_max_luns = DEF_MAX_LUNS;
>   static int sdebug_max_queue = SDEBUG_CANQUEUE;	/* per submit queue */
> +static unsigned int sdebug_max_segment_size = UINT_MAX;
>   static unsigned int sdebug_medium_error_start = OPT_MEDIUM_ERR_ADDR;
>   static int sdebug_medium_error_count = OPT_MEDIUM_ERR_NUM;
>   static int sdebug_ndelay = DEF_NDELAY;	/* if > 0 then unit is nanoseconds */
> @@ -7366,6 +7367,7 @@ module_param_named(lowest_aligned, sdebug_lowest_aligned, int, S_IRUGO);
>   module_param_named(lun_format, sdebug_lun_am_i, int, S_IRUGO | S_IWUSR);
>   module_param_named(max_luns, sdebug_max_luns, int, S_IRUGO | S_IWUSR);
>   module_param_named(max_queue, sdebug_max_queue, int, S_IRUGO | S_IWUSR);
> +module_param_named(max_segment_size, sdebug_max_segment_size, uint, S_IRUGO);
>   module_param_named(medium_error_count, sdebug_medium_error_count, int,
>   		   S_IRUGO | S_IWUSR);
>   module_param_named(medium_error_start, sdebug_medium_error_start, int,
> @@ -7449,6 +7451,7 @@ MODULE_PARM_DESC(lowest_aligned, "lowest aligned lba (def=0)");
>   MODULE_PARM_DESC(lun_format, "LUN format: 0->peripheral (def); 1 --> flat address method");
>   MODULE_PARM_DESC(max_luns, "number of LUNs per target to simulate(def=1)");
>   MODULE_PARM_DESC(max_queue, "max number of queued commands (1 to max(def))");
> +MODULE_PARM_DESC(max_segment_size, "max bytes in a single segment");
>   MODULE_PARM_DESC(medium_error_count, "count of sectors to return follow on MEDIUM error");
>   MODULE_PARM_DESC(medium_error_start, "starting sector number to return MEDIUM error");
>   MODULE_PARM_DESC(ndelay, "response delay in nanoseconds (def=0 -> ignore)");
> @@ -9539,7 +9542,6 @@ static const struct scsi_host_template sdebug_driver_template = {
>   	.sg_tablesize =		SG_MAX_SEGMENTS,
>   	.cmd_per_lun =		DEF_CMD_PER_LUN,
>   	.max_sectors =		-1U,
> -	.max_segment_size =	-1U,
>   	.module =		THIS_MODULE,
>   	.skip_settle_delay =	1,
>   	.track_queue_depth =	1,
> @@ -9566,6 +9568,7 @@ static int sdebug_driver_probe(struct device *dev)
>   	}
>   	hpnt->can_queue = sdebug_max_queue;
>   	hpnt->cmd_per_lun = sdebug_max_queue;
> +	hpnt->max_segment_size = sdebug_max_segment_size;
>   	if (!sdebug_clustering)
>   		hpnt->dma_boundary = PAGE_SIZE - 1;
>