Re: [PATCH 1/3] block: fix blk_rq_get_max_sectors() to flow more carefully

[Damien Le Moal <Damien.LeMoal@wdc.com>]

On 2020/09/14 23:52, Mike Snitzer wrote:
> On Sun, Sep 13 2020 at  8:43pm -0400,
> Damien Le Moal <Damien.LeMoal@wdc.com> wrote:
> 
>> On 2020/09/12 22:53, Ming Lei wrote:
>>> On Fri, Sep 11, 2020 at 05:53:36PM -0400, Mike Snitzer wrote:
>>>> blk_queue_get_max_sectors() has been trained for REQ_OP_WRITE_SAME and
>>>> REQ_OP_WRITE_ZEROES yet blk_rq_get_max_sectors() didn't call it for
>>>> those operations.
>>>
>>> Actually WRITE_SAME & WRITE_ZEROS are handled by the following if
>>> chunk_sectors is set:
>>>
>>>         return min(blk_max_size_offset(q, offset),
>>>                         blk_queue_get_max_sectors(q, req_op(rq)));
>>>  
>>>> Also, there is no need to avoid blk_max_size_offset() if
>>>> 'chunk_sectors' isn't set because it falls back to 'max_sectors'.
>>>>
>>>> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
>>>> ---
>>>>  include/linux/blkdev.h | 19 +++++++++++++------
>>>>  1 file changed, 13 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
>>>> index bb5636cc17b9..453a3d735d66 100644
>>>> --- a/include/linux/blkdev.h
>>>> +++ b/include/linux/blkdev.h
>>>> @@ -1070,17 +1070,24 @@ static inline unsigned int blk_rq_get_max_sectors(struct request *rq,
>>>>  						  sector_t offset)
>>>>  {
>>>>  	struct request_queue *q = rq->q;
>>>> +	int op;
>>>> +	unsigned int max_sectors;
>>>>  
>>>>  	if (blk_rq_is_passthrough(rq))
>>>>  		return q->limits.max_hw_sectors;
>>>>  
>>>> -	if (!q->limits.chunk_sectors ||
>>>> -	    req_op(rq) == REQ_OP_DISCARD ||
>>>> -	    req_op(rq) == REQ_OP_SECURE_ERASE)
>>>> -		return blk_queue_get_max_sectors(q, req_op(rq));
>>>> +	op = req_op(rq);
>>>> +	max_sectors = blk_queue_get_max_sectors(q, op);
>>>>  
>>>> -	return min(blk_max_size_offset(q, offset),
>>>> -			blk_queue_get_max_sectors(q, req_op(rq)));
>>>> +	switch (op) {
>>>> +	case REQ_OP_DISCARD:
>>>> +	case REQ_OP_SECURE_ERASE:
>>>> +	case REQ_OP_WRITE_SAME:
>>>> +	case REQ_OP_WRITE_ZEROES:
>>>> +		return max_sectors;
>>>> +	}>> +
>>>> +	return min(blk_max_size_offset(q, offset), max_sectors);
>>>>  }
>>>
>>> It depends if offset & chunk_sectors limit for WRITE_SAME & WRITE_ZEROS
>>> needs to be considered.
>>
>> That limit is needed for zoned block devices to ensure that *any* write request,
>> no matter the command, do not cross zone boundaries. Otherwise, the write would
>> be immediately failed by the device.
> 
> Thanks for the additional context, sorry to make you so concerned! ;)

No worries :)



-- 
Damien Le Moal
Western Digital Research