Re: [PATCH v7 1/3] bio: limit bio max size

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

On 4/18/21 10:49 PM, Changheun Lee wrote:
>>> @@ -167,6 +168,7 @@ void blk_queue_max_hw_sectors(struct request_queue *q, unsigned int max_hw_secto
>>>  	max_sectors = round_down(max_sectors,
>>>  				 limits->logical_block_size >> SECTOR_SHIFT);
>>>  	limits->max_sectors = max_sectors;
>>> +	limits->bio_max_bytes = max_sectors << SECTOR_SHIFT;
>>>  
>>>  	q->backing_dev_info->io_pages = max_sectors >> (PAGE_SHIFT - 9);
>>>  }
>>
>> Can the new shift operation overflow? If so, how about using
>> check_shl_overflow()?
> 
> Actually, overflow might be not heppen in case of physical device.
> But I modified as below. feedback about this.
> 
> @@ -168,6 +169,9 @@ void blk_queue_max_hw_sectors(struct request_queue *q, unsigned int max_hw_secto
>  				 limits->logical_block_size >> SECTOR_SHIFT);
>  	limits->max_sectors = max_sectors;
>  
> +	limits->bio_max_bytes = check_shl_overflow(max_sectors, SECTOR_SHIFT,
> +		&limits->bio_max_bytes) ? UINT_MAX : max_sectors << SECTOR_SHIFT;
> +
>  	q->backing_dev_info->io_pages = max_sectors >> (PAGE_SHIFT - 9);
>  }
>  EXPORT_SYMBOL(blk_queue_max_hw_sectors);

If no overflow occurs, check_shl_overflow() stores the result in the
memory location the third argument points at. So the above expression
can be simplified into the following:

if (check_shl_overflow(max_sectors, SECTOR_SHIFT, &limits->bio_max_bytes)) {
	limits->bio_max_bytes = UINT_MAX;
}

>>> diff --git a/include/linux/bio.h b/include/linux/bio.h
>>> index d0246c92a6e8..e5add63da3af 100644
>>> --- a/include/linux/bio.h
>>> +++ b/include/linux/bio.h
>>> @@ -106,6 +106,8 @@ static inline void *bio_data(struct bio *bio)
>>>  	return NULL;
>>>  }
>>>  
>>> +extern unsigned int bio_max_size(struct bio *bio);
>>
>> You may want to define bio_max_size() as an inline function in bio.h
>> such that no additional function calls are introduced in the hot path.
> 
> I tried, but it is not easy. because request_queue structure of blkdev.h
> should be referred in bio.h. I think it's not good to apply as a inline function.

Please don't worry about this. Inlining bio_max_size() is not a big
concern to me.

Thanks,

Bart.