On Fri, Sep 23, 2022 at 05:38:19PM +0200, Christoph Hellwig wrote:
>
>> +	else {
>> +		struct iovec fast_iov[UIO_FASTIOV];
>> +		struct iovec *iov = fast_iov;
>> +		struct iov_iter iter;
>> +
>> +		ret = import_iovec(rq_data_dir(req), ubuffer, bufflen,
>> +				UIO_FASTIOV, &iov, &iter);
>> +		if (ret < 0)
>>  			goto out;
>> +		ret = blk_rq_map_user_iov(q, req, NULL, &iter, GFP_KERNEL);
>> +		kfree(iov);
>> +	}
>
>While you touch this:  I think thi block of code would also be a good
>separate helper.  Maybe even in the block layer given the the scsi
>ioctl code and sg duplicate it, and already missed the fast_iov
>treatment due to the duplication.  Having this in a separate function
>is also nice to keep the fast_iov stack footprint isolated.

Totally agree on goodness. 
I think instead of new helper this seems suited to go inside
blk_rq_map_user_iov itself. That will make it symmetric to
blk_rq_map_user which also combines import + mapping.

But if I go that route now, I will have to alter parameters of
blk_rq_map_user_iov, and that will make it mandatory to change the
callers (scsi-ioctl, sg) too. Nothing hairy, but that means further
growth of unrelated elements in this series. Hope you agree that
separate series is much better, which I will post after this.

Will fold all other changes you pointed.