linux-kernel - Re: [PATCH 09/60] dm: dm.c: replace 'bio->bi_vcnt == 1' with !bio_multiple

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Message-ID: <CACVXFVNxwL1Dm9Mbf=YXpjV4X5G45Ed_TOE1A=wAORWkEdaHCA@mail.gmail.com>
Date:   Wed, 2 Nov 2016 15:56:41 +0800
From:   Ming Lei <tom.leiming@...il.com>
To:     Kent Overstreet <kent.overstreet@...il.com>
Cc:     Christoph Hellwig <hch@...radead.org>, Jens Axboe <axboe@...com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>,
        "maintainer:DEVICE-MAPPER (LVM)" <dm-devel@...hat.com>,
        Shaohua Li <shli@...nel.org>,
        "open list:SOFTWARE RAID (Multiple Disks) SUPPORT" 
        <linux-raid@...r.kernel.org>
Subject: Re: [PATCH 09/60] dm: dm.c: replace 'bio->bi_vcnt == 1' with !bio_multiple_segments

On Wed, Nov 2, 2016 at 11:09 AM, Kent Overstreet
<kent.overstreet@...il.com> wrote:
> On Mon, Oct 31, 2016 at 08:29:01AM -0700, Christoph Hellwig wrote:
>> On Sat, Oct 29, 2016 at 04:08:08PM +0800, Ming Lei wrote:
>> > Avoid to access .bi_vcnt directly, because it may be not what
>> > the driver expected any more after supporting multipage bvec.
>> >
>> > Signed-off-by: Ming Lei <tom.leiming@...il.com>
>>
>> It would be really nice to have a comment in the code why it's
>> even checking for multiple segments.
>
> Or ideally refactor the code to not care about multiple segments at all.

The check on 'bio->bi_vcnt == 1' is introduced in commit de3ec86dff160(dm:
don't start current request if it would've merged with the previous), which
fixed one performance issue.[1]

Looks the idea of the patch is to delay dispatching the rq if it
would've merged with previous request and the rq is small(single bvec).
I guess the motivation is to try to increase chance of merging with the delay.

But why does the code check on 'bio->bi_vcnt == 1'? Once the bio is
submitted, .bi_vcnt isn't changed any more and merging doesn't change
it too. So should the check have been on blk_rq_bytes(rq)?

Mike, please correct me if my understanding is wrong.


[1] https://www.redhat.com/archives/dm-devel/2015-March/msg00014.html


thanks,
Ming Lei