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From:   Lukas Bulwahn <lukas.bulwahn@gmail.com>
Date:   Wed, 10 May 2023 07:40:47 +0200
Message-ID: <CAKXUXMzA8GfJaWmdaszhrTtjMy5987oZ8AaBcoT6hNVvgCiZ-w@mail.gmail.com>
Subject: Re: Situation of CONFIG_BLK_WBT_MQ after commit b11d31ae01e6
 ("blk-wbt: remove unnecessary check in wbt_enable_default()")
To:     Yu Kuai <yukuai1@huaweicloud.com>
Cc:     Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        linux-block@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "yukuai (C)" <yukuai3@huawei.com>
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On Wed, May 10, 2023 at 5:24=E2=80=AFAM Yu Kuai <yukuai1@huaweicloud.com> w=
rote:
>
> Hi,
>
> =E5=9C=A8 2023/05/08 12:37, Lukas Bulwahn =E5=86=99=E9=81=93:
> > Dear Yu Kuai, dear Christoph, dear Jens,
> >
> >
> > The commit b11d31ae01e6 ("blk-wbt: remove unnecessary check in
> > wbt_enable_default()") removes the only reference to the config
> > BLK_WBT_MQ in the kernel tree.
> >
> > The commit comes with the statement "If CONFIG_BLK_WBT_MQ is disabled,
> > wbt_init() won't do anything.". The statement "If CONFIG_BLK_WBT is
> > disabled, wbt_init() won't do anything." (note: CONFIG_BLK_WBT vs.
> > CONFIG_BLK_WBT_MQ) is certainly true, but I do not see that "If
> > CONFIG_BLK_WBT_MQ is disabled, wbt_init() won't do anything.", but I
> > believe it would simply do what wbt_init() does with CONFIG_BLK_WBT
> > being enabled.
> >
> > Now, it seems that with this commit applied, the intended switch of
> > the config BLK_WBT_MQ is gone, and the config really now has no effect
> > at all.
> >
> > So, I am a bit puzzled:
> >
> > 1. Either the config BLK_WBT_MQ does have an effect somewhere, but one
> > cannot find its reference with 'git grep -i "BLK_WBT_MQ" .' --- so, my
> > investigation is just incomplete or wrong, or
> >
> > 2. We really do not need this further build config BLK_WBT_MQ beyond
> > the other configs already there --- then this config should just be
> > removed, or
> >
> > 3. the commit unintentionally broke the purpose of the config
> > BLK_WBT_MQ --- then this commit above should be reverted.
>
> Thanks for the report, it's the above case and it's my mistake.
> I will fix this.
>

Kuai, thanks for letting me know. Feel free to add a "Reported-by:
Lukas Bulwahn <lukas.bulwahn@gmail.com>" tag, and to include in the
list of recipients when you send out the fix to the mailing list.

I am glad to see that my investigation of kernel code is finding some
issues that we can fix.

Lukas