Re: [PATCH v6 8/8] null_blk: Support configuring the maximum segment size

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

On 6/13/23 09:44, Bart Van Assche wrote:
> On 6/12/23 15:17, Damien Le Moal wrote:
>> On 6/13/23 05:33, Bart Van Assche wrote:
>>> @@ -1283,7 +1293,8 @@ static int null_handle_rq(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;
>>
>> I am still confused by this change... Why is it necessary ? If max_segment_size
>> is set correctly, how can we ever get a BIO with a bvec length exceeding that
>> maximum ? If that is the case, aren't we missing a bio_split() somewhere ?
> 
> Hi Damien,
> 
> bio_split() enforces the max_sectors limit but not the max_segment_size 
> limit. __blk_rq_map_sg() enforces the max_segment_size limit. null_blk 
> does not call __blk_rq_map_sg(). Hence the above code to enforce the 
> max_segment_size limit.

OK. That is where I was confused :)
Thanks !

> 
> Thanks,
> 
> Bart.
> 

-- 
Damien Le Moal
Western Digital Research