Re: [PATCH 2/2] null_blk: Support configuring the maximum segment size

[Damien Le Moal <dlemoal@kernel.org>]

On 2026/03/23 13:29, Bart Van Assche wrote:
> Add support for configuring the maximum segment size. The maximum segment
> size may be set to a value smaller than the virtual memory page size.
> 
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Ming Lei <ming.lei@redhat.com>
> Cc: Damien Le Moal <damien.lemoal@opensource.wdc.com>
> Cc: Chaitanya Kulkarni <kch@nvidia.com>
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> ---
>  drivers/block/null_blk/main.c     | 13 +++++++++++++
>  drivers/block/null_blk/null_blk.h |  1 +
>  2 files changed, 14 insertions(+)
> 
> diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
> index 677ac829ef80..bf63431f6820 100644
> --- a/drivers/block/null_blk/main.c
> +++ b/drivers/block/null_blk/main.c
> @@ -169,6 +169,10 @@ static int g_max_sectors;
>  module_param_named(max_sectors, g_max_sectors, int, 0444);
>  MODULE_PARM_DESC(max_sectors, "Maximum size of a command (in 512B sectors)");
>  
> +static unsigned int g_max_segment_size = BLK_MAX_SEGMENT_SIZE;
> +module_param_named(max_segment_size, g_max_segment_size, int, 0444);
> +MODULE_PARM_DESC(max_segment_size, "Maximum size of a segment in bytes");

Maximum size of a DMA segment in bytes ?

(to be consistent with the code comment describing this. "segment" alone is not
clear...)

> +
>  static unsigned int nr_devices = 1;
>  module_param(nr_devices, uint, 0444);
>  MODULE_PARM_DESC(nr_devices, "Number of devices to register");
> @@ -450,6 +454,7 @@ NULLB_DEVICE_ATTR(home_node, uint, NULL);
>  NULLB_DEVICE_ATTR(queue_mode, uint, NULL);
>  NULLB_DEVICE_ATTR(blocksize, uint, NULL);
>  NULLB_DEVICE_ATTR(max_sectors, uint, NULL);
> +NULLB_DEVICE_ATTR(max_segment_size, uint, NULL);
>  NULLB_DEVICE_ATTR(irqmode, uint, NULL);
>  NULLB_DEVICE_ATTR(hw_queue_depth, uint, NULL);
>  NULLB_DEVICE_ATTR(index, uint, NULL);
> @@ -608,6 +613,7 @@ static struct configfs_attribute *nullb_device_attrs[] = {
>  	&nullb_device_attr_index,
>  	&nullb_device_attr_irqmode,
>  	&nullb_device_attr_max_sectors,
> +	&nullb_device_attr_max_segment_size,
>  	&nullb_device_attr_mbps,
>  	&nullb_device_attr_memory_backed,
>  	&nullb_device_attr_no_sched,
> @@ -805,6 +811,7 @@ static struct nullb_device *null_alloc_dev(void)
>  	dev->queue_mode = g_queue_mode;
>  	dev->blocksize = g_bs;
>  	dev->max_sectors = g_max_sectors;
> +	dev->max_segment_size = g_max_segment_size;
>  	dev->irqmode = g_irqmode;
>  	dev->hw_queue_depth = g_hw_queue_depth;
>  	dev->blocking = g_blocking;
> @@ -1251,6 +1258,9 @@ static blk_status_t null_transfer(struct nullb *nullb, struct page *page,
>  	unsigned int valid_len = len;
>  	void *p;
>  
> +	WARN_ONCE(len > dev->max_segment_size, "%u > %u\n", len,
> +		  dev->max_segment_size);
> +
>  	p = kmap_local_page(page) + off;
>  	if (!is_write) {
>  		if (dev->zoned) {
> @@ -1298,6 +1308,8 @@ static blk_status_t null_handle_data_transfer(struct nullb_cmd *cmd,
>  	spin_lock_irq(&nullb->lock);
>  	rq_for_each_segment(bvec, rq, iter) {
>  		len = bvec.bv_len;
> +		len = min(bvec.bv_len, nullb->dev->max_segment_size);
> +		bvec.bv_len = len;
>  		if (transferred_bytes + len > max_bytes)
>  			len = max_bytes - transferred_bytes;
>  		err = null_transfer(nullb, bvec.bv_page, len, bvec.bv_offset,
> @@ -1961,6 +1973,7 @@ static int null_add_dev(struct nullb_device *dev)
>  		.logical_block_size	= dev->blocksize,
>  		.physical_block_size	= dev->blocksize,
>  		.max_hw_sectors		= dev->max_sectors,
> +		.max_segment_size	= dev->max_segment_size,
>  		.dma_alignment		= 1,
>  	};
>  
> diff --git a/drivers/block/null_blk/null_blk.h b/drivers/block/null_blk/null_blk.h
> index 6c4c4bbe7dad..43dc47789718 100644
> --- a/drivers/block/null_blk/null_blk.h
> +++ b/drivers/block/null_blk/null_blk.h
> @@ -93,6 +93,7 @@ struct nullb_device {
>  	unsigned int queue_mode; /* block interface */
>  	unsigned int blocksize; /* block size */
>  	unsigned int max_sectors; /* Max sectors per command */
> +	unsigned int max_segment_size; /* Max size of a single DMA segment. */
>  	unsigned int irqmode; /* IRQ completion handler */
>  	unsigned int hw_queue_depth; /* queue depth */
>  	unsigned int index; /* index of the disk, only valid with a disk */


-- 
Damien Le Moal
Western Digital Research