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

[Bart Van Assche <bvanassche@acm.org>]

On 3/29/26 7:38 AM, Ming Lei wrote:
> 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?

Your commit 889c57066cee ("block: make segment size limit workable for >
4K PAGE_SIZE") adds support for devices with a maximum segment size
limit that is less than the virtual memory page size. With 
BLK_MIN_SEGMENT_SIZE == 4096, it is not possible to test the code paths
added by this commit if PAGE_SIZE == 4096. Allowing a segment size limit
that is less than 4096 bytes makes it possible to test the new code
paths on systems with PAGE_SIZE == 4096, e.g. an x86 VM.

>>   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.

I will drop the previous patch.

Thanks,

Bart.