Re: [PATCH v3 4/6] block: Reduce the minimum value for the maximum DMA segment size

[Ming Lei <ming.lei@redhat.com>]

On Fri, Mar 27, 2026 at 02:13:44PM -0700, Bart Van Assche wrote:
> All block devices that are supported by the Linux kernel have a DMA engine
> that supports DMA segments of 4 KiB or larger. Allow smaller DMA segment
> sizes because these are useful for block layer testing. Reject values below

Can you share why/what it is useful just for test purpose?

If there isn't such real device with 512 segment size, why do we want this
change for covering it?

> 512 because such values would result in an excessive number of DMA
> segments. Move the BLK_MIN_SEGMENT_SIZE definition into <linux/blkdev.h>.
> This will allow the BLK_MIN_SEGMENT_SIZE constant to be used in the
> null_blk and scsi_debug drivers.
> 
> The only code affected by this change is the following code:
> 
> 	if (WARN_ON_ONCE(lim->max_segment_size < BLK_MIN_SEGMENT_SIZE))
> 		return -EINVAL;
> 
> Cc: Ming Lei <ming.lei@redhat.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> ---
>  block/blk.h            | 1 -
>  include/linux/blkdev.h | 1 +
>  2 files changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/blk.h b/block/blk.h
> index 103cb1d0b9cb..b30ff8db3cac 100644
> --- a/block/blk.h
> +++ b/block/blk.h
> @@ -23,7 +23,6 @@ struct elv_change_ctx;
>  #define BLK_DEF_MAX_SECTORS_CAP	(SZ_4M >> SECTOR_SHIFT)
>  
>  #define	BLK_DEV_MAX_SECTORS	(LLONG_MAX >> 9)
> -#define	BLK_MIN_SEGMENT_SIZE	4096
>  
>  /* Max future timer expiry for timeouts */
>  #define BLK_MAX_TIMEOUT		(5 * HZ)
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index b8e7f42aee71..109d5fa5e190 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -1311,6 +1311,7 @@ static inline bool bdev_is_partition(struct block_device *bdev)
>  enum blk_default_limits {
>  	BLK_MAX_SEGMENTS	= 128,
>  	BLK_SAFE_MAX_SECTORS	= 255,
> +	BLK_MIN_SEGMENT_SIZE	= 512,
>  	BLK_MAX_SEGMENT_SIZE	= 65536,
>  	BLK_SEG_BOUNDARY_MASK	= 0xFFFFFFFFUL,

This change actually becomes not consistent with previous patch, in which ->seg_boundary_mask
can be 4095.


Thank,
Ming