Re: [PATCH v1 4/6] block: check bio_mergeable() early before merging

[Jeff Moyer]

Ming Lei <ming.lei@xxxxxxxxxxxxx> writes:

> On Mon, Oct 19, 2015 at 11:27 PM, Jeff Moyer <jmoyer@xxxxxxxxxx> wrote:
>> Ming Lei <ming.lei@xxxxxxxxxxxxx> writes:
>>
>>> After bio splitting is introduced, one bio can be splitted
>>> and it is marked as NOMERGE because it is too fat to be merged,
>>> so check bio_mergeable() earlier to avoid to try to merge it
>>> unnecessarily.
>>>
>>> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxxxxx>
>>
>> Reviewed-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
>>
>> Ming, do you think we should also add a check in blk_attempt_plug_merge?
>
> No, because 'request_count' need to be figured out for automatic flush plug.

Interesting.  See my follow-up to patch 5.  :)

-Jeff

>
> Thanks,
>
>>
>> -Jeff
>>
>>> ---
>>>  block/elevator.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/block/elevator.c b/block/elevator.c
>>> index 84d6394..c3555c9 100644
>>> --- a/block/elevator.c
>>> +++ b/block/elevator.c
>>> @@ -420,7 +420,7 @@ int elv_merge(struct request_queue *q, struct request **req, struct bio *bio)
>>>        *      noxmerges: Only simple one-hit cache try
>>>        *      merges:    All merge tries attempted
>>>        */
>>> -     if (blk_queue_nomerges(q))
>>> +     if (blk_queue_nomerges(q) || !bio_mergeable(bio))
>>>               return ELEVATOR_NO_MERGE;
>>>
>>>       /*
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