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

[Damien Le Moal]

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 at 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,
> Ming
> 
> 


-- 
Damien Le Moal
Western Digital Research