linux-kernel - Re: [PATCH 2/4] block: check virt boundary in bio_will

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Message-ID: <56C2349D.4020708@dev.mellanox.co.il>
Date:	Mon, 15 Feb 2016 22:27:09 +0200
From:	Sagi Grimberg <sagig@....mellanox.co.il>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-block@...r.kernel.org, Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH 2/4] block: check virt boundary in bio_will_gap()


>>> 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
payloads...