Re: [PATCH 2/4] block: check virt boundary in bio_will_gap()

[Ming Lei]

On Tue, Feb 16, 2016 at 4:27 AM, Sagi Grimberg <sagig@xxxxxxxxxxxxxxxxxx> wrote:
>
>>>> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
>>>> index 4571ef1..b8ff6a3 100644
>>>> --- a/include/linux/blkdev.h
>>>> +++ b/include/linux/blkdev.h
>>>> @@ -1388,7 +1388,7 @@ static inline bool bvec_gap_to_prev(struct
>>>> request_queue *q,
>>>>    static inline bool bio_will_gap(struct request_queue *q, struct bio
>>>> *prev,
>>>>                           struct bio *next)
>>>>    {
>>>> -       if (!bio_has_data(prev))
>>>> +       if (!bio_has_data(prev) || !queue_virt_boundary(q))
>>>>    bio_integrity_add_page              return false;
>>>
>>>
>>>
>>> Can we not do that?
>>
>>
>> Given there are only 3 drivers which set virt boundary, I think
>> it is reasonable to do that.
>
>
> 3 drivers that are really performance critical. I don't think we
> should add optimized branching for some of the drivers especially
> when the drivers that do set virt_boundary *really* care about latency.
>
>>> bvec_gap_to_prev is already checking the virt_boundary and I'd sorta
>>> like to keep the motivation to optimize bio_get_last_bvec() to be O(1).
>>
>>
>> Currently the approaches I thought of still need to iterate bvec by bvec,
>> not sure if O(1) can be reached easily, but I am happy to discuss the
>> optimized implementation.
>
>
> Me too. Note that I don't mind if the bio split code won't be optimized,
> but I do want req_gap_back_merge/req_gap_front_merge to be...
>
> Also, are the bvec_gap_to_prev usages in bio_add_pc_page and
> bio_integrity_add_page safe? I didn't test this stuff with integrity

Yes, because both are non-cloned bvec table.

> payloads...

Thanks,