Re: [PATCH V3] block: fix the DISCARD request merge

[jianchao.wang]

On 10/26/18 4:07 PM, Christoph Hellwig wrote:
> On Fri, Oct 26, 2018 at 04:05:00PM +0800, jianchao.wang wrote:
>> Would anyone please take a look at this ?
> 
> I did take a look and reply to it..

Oh sorry, I missed it in my email client.
Got it from the https://lkml.org/lkml/2018/10/24/491
And we could also have a blk_merge_req_ok there. :)

Thanks
Jianchao
> 
>>
>> Thanks in advance.
>> Jianchao
>>
>> On 10/24/18 11:07 AM, Jianchao Wang wrote:
>>> There are two cases when handle DISCARD merge.
>>> If max_discard_segments == 1, the bios/requests need to be contiguous
>>> to merge. If max_discard_segments > 1, it takes every bio as a range
>>> and different range needn't to be contiguous.
>>>
>>> But now, attempt_merge screws this up. It always consider contiguity
>>> for DISCARD for the case max_discard_segments > 1 and cannot merge
>>> contiguous DISCARD for the case max_discard_segments == 1, because
>>> rq_attempt_discard_merge always returns false in this case.
>>> This patch fixes both of the two cases above.
>>>
>>> Signed-off-by: Jianchao Wang <jianchao.w.wang@xxxxxxxxxx>
>>> ---
>>>
>>> V3:
>>>  - Introduce blk_discard_mergable into attempt_merge and
>>>    blk_try_merge.
>>>  - Some comment changes.
>>>
>>> V2:
>>>  - Add max_discard_segments > 1 checking in attempt_merge.
>>>  - Change patch title and comment.
>>>  - Add more comment in attempt_merge
>>>
>>>  block/blk-merge.c | 34 ++++++++++++++++++++++++----------
>>>  1 file changed, 24 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/block/blk-merge.c b/block/blk-merge.c
>>> index 42a4674..b258de0 100644
>>> --- a/block/blk-merge.c
>>> +++ b/block/blk-merge.c
>>> @@ -714,6 +714,22 @@ static void blk_account_io_merge(struct request *req)
>>>  		part_stat_unlock();
>>>  	}
>>>  }
>>> +/*
>>> + * Two cases of handling DISCARD merge:
>>> + * If max_discard_segments > 1, the driver takes every bio
>>> + * as a range and send them to controller together. The ranges
>>> + * needn't to be contiguous.
>>> + * Otherwise, the bios/requests will be handled as same as
>>> + * others which should be contiguous.
>>> + */
>>> +static inline bool blk_discard_mergable(struct request *req)
>>> +{
>>> +	if (req_op(req) == REQ_OP_DISCARD &&
>>> +	    queue_max_discard_segments(req->q) > 1)
>>> +		return true;
>>> +	else
>>> +		return false;
>>> +}
>>>  
>>>  /*
>>>   * For non-mq, this has to be called with the request spinlock acquired.
>>> @@ -731,12 +747,6 @@ static struct request *attempt_merge(struct request_queue *q,
>>>  	if (req_op(req) != req_op(next))
>>>  		return NULL;
>>>  
>>> -	/*
>>> -	 * not contiguous
>>> -	 */
>>> -	if (blk_rq_pos(req) + blk_rq_sectors(req) != blk_rq_pos(next))
>>> -		return NULL;
>>> -
>>>  	if (rq_data_dir(req) != rq_data_dir(next)
>>>  	    || req->rq_disk != next->rq_disk
>>>  	    || req_no_special_merge(next))
>>> @@ -760,11 +770,16 @@ static struct request *attempt_merge(struct request_queue *q,
>>>  	 * counts here. Handle DISCARDs separately, as they
>>>  	 * have separate settings.
>>>  	 */
>>> -	if (req_op(req) == REQ_OP_DISCARD) {
>>> +
>>> +	if (blk_discard_mergable(req)) {
>>>  		if (!req_attempt_discard_merge(q, req, next))
>>>  			return NULL;
>>> -	} else if (!ll_merge_requests_fn(q, req, next))
>>> +	} else if (blk_rq_pos(req) + blk_rq_sectors(req) == blk_rq_pos(next)) {
>>> +		if (!ll_merge_requests_fn(q, req, next))
>>> +			return NULL;
>>> +	} else {
>>>  		return NULL;
>>> +	}
>>>  
>>>  	/*
>>>  	 * If failfast settings disagree or any of the two is already
>>> @@ -888,8 +903,7 @@ bool blk_rq_merge_ok(struct request *rq, struct bio *bio)
>>>  
>>>  enum elv_merge blk_try_merge(struct request *rq, struct bio *bio)
>>>  {
>>> -	if (req_op(rq) == REQ_OP_DISCARD &&
>>> -	    queue_max_discard_segments(rq->q) > 1)
>>> +	if (blk_discard_mergable(rq))
>>>  		return ELEVATOR_DISCARD_MERGE;
>>>  	else if (blk_rq_pos(rq) + blk_rq_sectors(rq) == bio->bi_iter.bi_sector)
>>>  		return ELEVATOR_BACK_MERGE;
>>>
> ---end quoted text---
>