linux-kernel - Situation of CONFIG_BLK_WBT_MQ after commit b11d31ae01e6 ("blk-wbt: remove unnecessary check in wbt_enable

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Message-ID: <CAKXUXMzfKq_J9nKHGyr5P5rvUETY4B-fxoQD4sO+NYjFOfVtZA@mail.gmail.com>
Date:   Mon, 8 May 2023 06:37:01 +0200
From:   Lukas Bulwahn <lukas.bulwahn@...il.com>
To:     Yu Kuai <yukuai3@...wei.com>, Christoph Hellwig <hch@....de>,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Situation of CONFIG_BLK_WBT_MQ after commit b11d31ae01e6 ("blk-wbt:
 remove unnecessary check in wbt_enable_default()")

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.

I would be happy if you could provide some guidance on what is the
situation with config BLK_WBT_MQ.

Currently, I am guessing it is option 2 and the config BLK_WBT_MQ
simply can be deleted, but I am really unsure. Probably, this build
option is not used by many people and its effect is hardly noticed, if
one does not specifically check for that in the running system.

Best regards,

Lukas